为什么qml仅支持矩形，不支持多边形、圆形、直线等图元？

Qt 5.12.2 (x86_64-little_endian-lp64 shared (dynamic) release build; by Clang 10.0.0 (clang-1000.11.45.5) (Apple)) on "cocoa" OS: macOS High Sierra (10.13) [darwin version 17.7.0]   Architecture: x86_64; features: SSE2 SSE3 SSSE3 SSE4.1 SSE4.2 AVX AVX2   Environment:   Features: QT_NO_EXCEPTIONS QT_NO_OPENSSL   Library info: PrefixPath: /Users/wangshuai/Public/Qt5.12.2/Qt Creator.app/Contents DocumentationPath: /Users/wangshuai/Public/Qt5.12.2/Qt Creator.app/Contents/doc HeadersPath: /Users/wangshuai/Public/Qt5.12.2/Qt Creator.app/Contents/include LibrariesPath: /Users/wangshuai/Public/Qt5.12.2/Qt Creator.app/Contents/lib LibraryExecutablesPath: /Users/wangshuai/Public/Qt5.12.2/Qt Creator.app/Contents/libexec BinariesPath: /Users/wangshuai/Public/Qt5.12.2/Qt Creator.app/Contents/MacOS PluginsPath: /Users/wangshuai/Public/Qt5.12.2/Qt Creator.app/Contents/PlugIns ImportsPath: /Users/wangshuai/Public/Qt5.12.2/Qt Creator.app/Contents/Imports/qtquick1 Qml2ImportsPath: /Users/wangshuai/Public/Qt5.12.2/Qt Creator.app/Contents/Imports/qtquick2 ArchDataPath: /Users/wangshuai/Public/Qt5.12.2/Qt Creator.app/Contents DataPath: /Users/wangshuai/Public/Qt5.12.2/Qt Creator.app/Contents TranslationsPath: /Users/wangshuai/Public/Qt5.12.2/Qt Creator.app/Contents/translations ExamplesPath: /Users/wangshuai/Public/Qt5.12.2/Qt Creator.app/Contents/examples TestsPath: /Users/wangshuai/Public/Qt5.12.2/Qt Creator.app/Contents/tests SettingsPath: /Users/wangshuai/Public/Qt5.12.2/Qt Creator.app/Contents   Standard paths [*...* denote writable entry]: DesktopLocation: "Desktop" */Users/wangshuai/Desktop* DocumentsLocation: "Documents" */Users/wangshuai/Documents* FontsLocation: "Fonts" */Users/wangshuai/Library/Fonts* /Library/Fonts /System/Library/Fonts ApplicationsLocation: "Applications" */Users/wangshuai/Applications* /Applications MusicLocation: "Music" */Users/wangshuai/Music* MoviesLocation: "Movies" */Users/wangshuai/Movies* PicturesLocation: "Pictures" */Users/wangshuai/Pictures* TempLocation: "Temporary Items" */private/var/folders/_7/hzt4cp6s0q74k71lxv36_4mc0000gn/T* HomeLocation: "Home" */Users/wangshuai* AppLocalDataLocation: "Application Support" */Users/wangshuai/Library/Application Support/QtProject/qtdiag* /Library/Application Support/QtProject/qtdiag /Users/wangshuai/Public/Qt5.12.2/Qt Creator.app/Contents/Resources CacheLocation: "Caches" */Users/wangshuai/Library/Caches/QtProject/qtdiag* /Library/Caches/QtProject/qtdiag GenericDataLocation: "Application Support" */Users/wangshuai/Library/Application Support* /Library/Application Support RuntimeLocation: "Application Support" */Users/wangshuai/Library/Application Support* ConfigLocation: "Preferences" */Users/wangshuai/Library/Preferences* DownloadLocation: "Downloads" */Users/wangshuai/Downloads* GenericCacheLocation: "Caches" */Users/wangshuai/Library/Caches* /Library/Caches /System/Library/Caches GenericConfigLocation: "Preferences" */Users/wangshuai/Library/Preferences* AppDataLocation: "Application Support" */Users/wangshuai/Library/Application Support/QtProject/qtdiag* /Library/Application Support/QtProject/qtdiag /Users/wangshuai/Public/Qt5.12.2/Qt Creator.app/Contents/Resources AppConfigLocation: "Preferences" */Users/wangshuai/Library/Preferences/QtProject/qtdiag*   File selectors (increasing order of precedence): zh_CN unix darwin mac osx macos   Network: Using "Secure Transport, macOS High Sierra (10.13)", version: 0x0   Platform capabilities: ThreadedPixmaps OpenGL BufferQueueingOpenGL WindowMasks MultipleWindows ApplicationState ForeignWindows NonFullScreenWindows NativeWidgets WindowManagement RasterGLSurface ApplicationIcon   Style hints: mouseDoubleClickInterval: 400 mousePressAndHoldInterval: 800 startDragDistance: 10 startDragTime: 500 startDragVelocity: 0 keyboardInputInterval: 400 keyboardAutoRepeatRate: 30 cursorFlashTime: 1000 showIsFullScreen: 0 showIsMaximized: 0 passwordMaskDelay: 0 passwordMaskCharacter: U+2022 fontSmoothingGamma: 2 useRtlExtensions: 0 setFocusOnTouchRelease: 0 tabFocusBehavior: Qt::TabFocusBehavior(3) singleClickActivation: 0   Additional style hints (QPlatformIntegration): ReplayMousePressOutsidePopup: 1   Theme: Platforms requested : cocoa available : Styles requested : macintosh available : macintosh,Windows,Fusion System font : ".SF NS Text" 13 Native file dialog Native color dialog Native font dialog   Fonts: General font : ".SF NS Text" 13 Fixed font : "Monaco" 10 Title font : ".SF NS Text" 13 Smallest font: ".SF NS Text" 9   Palette: QPalette::WindowText: #ff000000 QPalette::Button: #ffececec QPalette::Light: #ffffffff QPalette::Midlight: #fff5f5f5 QPalette::Dark: #ffbfbfbf QPalette::Mid: #ffa9a9a9 QPalette::Text: #ff000000 QPalette::BrightText: #ffffffff QPalette::ButtonText: #ff000000 QPalette::Base: #ffffffff QPalette::Window: #ffececec QPalette::Shadow: #ff000000 QPalette::Highlight: #ffb2d7ff QPalette::HighlightedText: #ff000000 QPalette::Link: #ff0000ff QPalette::LinkVisited: #ffff00ff QPalette::AlternateBase: #fff5f5f5 QPalette::NoRole: #ff000000 QPalette::ToolTipBase: #ffe8e8e8 QPalette::ToolTipText: #ff000000 QPalette::PlaceholderText: #80000000   Screens: 1, High DPI scaling: inactive # 0 " Plugin information:   + Android 4.9.0 + AutoTest 4.9.0 AutotoolsProjectManager 4.9.0 BareMetal 4.9.0 + Bazaar 4.9.0 Beautifier 4.9.0 + BinEditor 4.9.0 + Bookmarks 4.9.0 + CMakeProjectManager 4.9.0 + CVS 4.9.0 + ClangCodeModel 4.9.0 ClangFormat 4.9.0 + ClangTools 4.9.0 + ClassView 4.9.0 ClearCase 4.9.0 + CodePaster 4.9.0 CompilationDatabaseProjectManager 4.9.0 + Core 4.9.0 + CppEditor 4.9.0 + CppTools 4.9.0 Cppcheck 4.9.0 + Debugger 4.9.0 + Designer 4.9.0 + DiffEditor 4.9.0 EmacsKeys 4.9.0 + FakeVim 4.9.0 + GLSLEditor 4.9.0 + GenericProjectManager 4.9.0 + Git 4.9.0 HelloWorld 4.9.0 + Help 4.9.0 + ImageViewer 4.9.0 + Ios 4.9.0 LanguageClient 4.9.0 + Macros 4.9.0 + Mercurial 4.9.0 + ModelEditor 4.9.0 Nim 4.9.0 + PerfProfiler 4.9.0 Perforce 4.9.0 + ProjectExplorer 4.9.0 + PythonEditor 4.9.0 + QbsProjectManager 4.9.0 + QmakeProjectManager 4.9.0 + QmlDesigner 4.9.0 + QmlJSEditor 4.9.0 + QmlJSTools 4.9.0 + QmlPreview 4.9.0 + QmlProfiler 4.9.0 + QmlProjectManager 4.9.0 + Qnx 4.9.0 + QtSupport 4.9.0 + RemoteLinux 4.9.0 + ResourceEditor 4.9.0 + ScxmlEditor 4.9.0 SilverSearcher 4.9.0 + Subversion 4.9.0 + TaskList 4.9.0 + TextEditor 4.9.0 Todo 4.9.0 + UpdateInfo 4.9.0 + Valgrind 4.9.0 + VcsBase 4.9.0 + Welcome 4.9.0 WinRt 4.9.0   Qt Creator 4.9.0 Based on Qt 5.12.2 (Clang 10.0 (Apple), 64 bit) From revision 7885bc899f Built on Apr 11 2019 21:12:12