Improve high-DPI auto-testing

Goal
We'd like to Improve auto-test coverage of Qts high-DPI support, including CI regression testing. This requires running auto-tests in environments that does not have physical screens, by simulating (multiple) virtual screens:

Changes

• Add support for multiple virtual screens to the offscreen platform plugin, with settable properties such as geometry and DPI: https://codereview.qt-project.org/c/qt/qtbase/+/290632
• Write auto-tests for relevant API in QWindow, testing the QHighDPIScaling implementation and its usage by the QWindow implementation: https://codereview.qt-project.org/c/qt/qtbase/+/290633/1

Mock testing with the offscreen platform plugin overview

(as implemented in the changes above)

The offscreen platform plugin now supports taking a config file which specifies the virtual screen configuration: (example)

{
    “screens”: [
        {
        "name", "screen-1",
        "x", 0,
        "y", 0,
        "width", 640,
        "height", 480,
        "dpi", 96,
        "dpr", 1,
        },
        …
    ]
}

The config file path is specified using platform plugin arguments:

-platform offscfreen:configfile=/path/to/file

This enables two use cases:

• Running existing tests on any mock configuration. E.g, can tst_QWindow handle two screens with different DPI.
• Writing new tests witch instantiates different mock configurations, and then verifies that a specific Qt API/feature works as expected. (In practice, this is done by re-creating the QApplication a new config file.)

Future Work

• How can this be configured from e.g. a CI system script? Support specifying the config file by environment variable as well?
• Support events: "screen disconnected", "DPI changed".
  • sepcify events/timing using JSON config file.
  • Add QTestLib API for e.g. setting screen configuration.
• Can this be used by applications?

Limitations:

• Platform plugin code is not tested - we should implement as much in QtGui as possible
• We risk tailoring our tests to our the offscreen platform plugin implementation. Remember to test on actual platforms as well.