opengl can't upload R8 texture

change 'QSG_RHI_BACKEND' to 'vulkan' in main, we can see the color changes from 0 to 255,
but if use 'opengl', always black.

based on official example for qsg, I added uploadtexture code section in rendernode.

ubuntu 22.04, official qt6.5.1, intel gpu
I tried official gpu driver and https://ppa.launchpadcontent.net/oibaf/graphics-drivers/ubuntu/, both are same.

qt.scenegraph.general: threaded render loop
qt.scenegraph.general: Using sg animation driver
qt.scenegraph.general: Animation Driver: using vsync: 16.68 ms
qt.scenegraph.general: Using sg animation driver
qt.scenegraph.general: Animation Driver: using vsync: 16.68 ms
qt.scenegraph.general: Creating QRhi with backend OpenGL for window 0x555555660b40 (wflags 0x1)
  Graphics API debug/validation layers: 0
  Debug markers: 0
  Prefer software device: 0
  Shader/pipeline cache collection: 1
qt.rhi.general: Created OpenGL context QSurfaceFormat(version 4.6, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8, samples 0, swapBehavior QSurfaceFormat::DefaultSwapBehavior, swapInterval 1, colorSpace QColorSpace(), profile  QSurfaceFormat::CompatibilityProfile)
qt.rhi.general: OpenGL VENDOR: Intel RENDERER: Mesa Intel(R) UHD Graphics 770 (ADL-S GT1) VERSION: 4.6 (Compatibility Profile) Mesa 23.3~git2307250600.75452f~oibaf~j (git-75452f6 2023-07-25 jammy-oi
qt.scenegraph.general: Created QRhi 0x7fffc8005640 for window 0x555555660b40
qt.scenegraph.general: Attempting to seed pipeline cache for QRhi 0x7fffc8005640 from '/home/ll/.cache/otb/qtpipelinecache-x86_64-little_endian-lp64/qqpc_opengl'
qt.rhi.general: Seeded pipeline cache with 2 program binaries
qt.scenegraph.general: MSAA sample count for the swapchain is 1. Alpha channel requested = yes.
qt.scenegraph.general: rhi texture atlas dimensions: 1024x512