Re-add ability to switch render modes under Qt 6

[Cuperino]

Fixes #880, where QSG_VISUALIZE overdraw, and other modes, were not working under Plasma. It does this by re-enabling RenderModeRequest::apply() when using the OpenGL variants or the Software renderer. Other modes, such as Vulkan, are yet to be implemented.

Testing was done under Plasma Wayland, by combining:

• setting QSG_RHI_BACKEND to opengl, vulkan, and software
• setting QSG_RHI_PREFER_SOFTWARE_RENDERER to 0 and 1

[Cuperino]

quickinspector test is failing on Windows because of the following reasons:

• The overlay that states "DirectX is not supported yet, please use OpenGL or Software backend" dims color values, making color compare fail. A solution for this is to make this overlay a little smaller, so we can get a glimpse of the original colors from the corners we pick colors from in this test.
• The output image's canvas is scaled according to display scaling, but the contents of the image itself remain at its original, non-scaled, resolution. There's a slim chance, that I roughly estimate to approximate 1/100, that the image is scaled to fit the scaled canvas, and the test passes. Most of the time however, the test will fail with display scaling on.

Qt 5 out: note resolution of 232x100, with the image staying at 100x00

Qt 6 out: note resolution of 232x200, and how the image stays at 100x00

By solving the first problem, the one with the presence of an overlay, we get the following image:

For the test to pass on a system with display scaling, the image should instead produce this output that only happens rarely:

[Cuperino]

From inspecting quickscreengrabber.cpp:810, I suspect this could potentially be a bug with the grabWindow() function in Qt 6.

Edit: It could also be a race condition between the image signaling being captured and it being scaled by Qt.

[Waqar144]

A solution for this is to make this overlay a little smaller, so we can get a glimpse of the original colors

Yeah, but we don't support DirectX/Vulkan so making the test pass is meaningless and misguiding. I would rather skip it.

[Cuperino]

A solution for this is to make this overlay a little smaller, so we can get a glimpse of the original colors

Yeah, but we don't support DirectX/Vulkan so making the test pass is meaningless and misguiding. I would rather skip it.

Here the image behind the overlay wasn't loading at the correct scale, so I would leave the scale fix and remove the change to the overlay.

See:

0e9cfa3
grabbedFrame.image is sized according to display scaling, but
the contents of the image itself remain at their original,
non-scaled, resolution. There's a slim chance that the image is
scaled to fit the scaled canvas, and the quickinspectortest
passes. Most of the times, however, the image won't be scaled
to fit the canvas.

In this change we manually scale the image according to
devicePixelRatio to fit the image's canvas.

Alternatively, we could leave both, and use the test to catch when the image from the window grab loads with 4x scale instead of 2x. In other words, we could use the test to find out when the bug with the scaling factor and window.grabImage gets fixed in a future Qt version.

Right now the only test failing appears to be quickinspectortest when OpenGL is used Windows.
@Waqar144, Is OpenGL use supported on Windows?

[Waqar144]

@Waqar144, Is OpenGL use supported on Windows?

Afaik, yes. It should work on windows