Raspberry 4/5 colorspace issue on wayland

[IcoDeveloper]

• Platform:
  Linux raspi5-1 6.1.0-rpi8-rpi-2712 #1 SMP PREEMPT Debian 1:6.1.73-1+rpt1 (2024-01-25) aarch64 GNU/Linux
  (Raspberry OS bookworm)
  pi5 w/4GB, pi4 w/2GB

• MythTV version:
  master

• Package version:
  built from source and mythtv-light

• Component:
  frontend

What steps will reproduce the bug?

Start frontend

How often does it reproduce? Is there a required condition?

always when running on wayland.

What is the expected behaviour?

Mythtv display using a 24-bit display surface (RGBA 8:8:8) which shows smooth color gradients

What do you see instead?

Mythtv display using a 16-bit display surface (RGBA 5:6:5) which shows banding on color gradients; quite noticeable on mythcenter theme for example.
Recording / Video playback is also affected

mythfrontend shows info on the Qt OpenGL surface used in the log and console. expected (and seen when using X11)
CoreContext mythrenderopengl.cpp:430 (DebugFeatures) - OpenGL: Qt OpenGL surface : RGBA: 8:8:8:0 Depth: 0 Stencil: 0

Seen when using wayland:
CoreContext mythrenderopengl.cpp:430 (DebugFeatures) - OpenGL: Qt OpenGL surface : RGBA: 5:6:5:0 Depth: 0 Stencil: 0

Additional information

VLC manages to display videos using full 24-bit colorspace in the same gui session, so it doesn't seem to be a system constraint

[IcoDeveloper]

I can now confirm that requesting a 24-bit surface works under wayland. However, putting the code in the obvious place in mythrenderopengl.cpp didn't work and resulted in makeCurrent returning eglError 3009.
I suspect the problem is that an openGL context already exists at that point and the default format should be modified before creating a context.

changing the default SurfaceFormat in mythpainterwindowopengl.cpp right at the beginning does work however.

MythPainterWindowOpenGL::MythPainterWindowOpenGL(MythMainWindow *MainWin)
  : MythPainterWindow(MainWin),
    m_parent(MainWin)
{
    bool ok;
    auto envBitsPerPixel = qEnvironmentVariable("MYTHTV_BITS_COLOR");
    int bitsPerPixel;
    if (!envBitsPerPixel.isEmpty())
    {
        bitsPerPixel = envBitsPerPixel.toInt(&ok);
        if (ok)
        {
            LOG(VB_GENERAL, LOG_INFO, LOC + QString("Setting default SurfaceFormat to %1 bits/color").arg(bitsPerPixel));
            QSurfaceFormat defaultFormat;
            defaultFormat.setRedBufferSize(bitsPerPixel);
            defaultFormat.setGreenBufferSize(bitsPerPixel);
            defaultFormat.setBlueBufferSize(bitsPerPixel);
            QSurfaceFormat::setDefaultFormat(defaultFormat);
        }
    }

Starting mythfrontent without the environment variable gets the same RGBA 5:6:5 Surface as before, but
MYTHTV_BITS_COLOR=8 mythfrontend
results in RGBA 8:8:8 and smooth gradients both in the menus and in playback.
This is obviously not intended as an actual fix for the issue but does show that a workaround is feasible