Feature/Qt6 - Fix dropperSampler not working correctly

[pbergeron-adsk]

Summarize your change.

The dropperSampler tool in RV was not working correctly anymore in the qt6 branch. It had a strange offset (horizontally and vertically) because the OpenGL function glReadPixels was reading from the wrong context and the wrong framebuffer object.

in annotate_mode.mu, the method dropperSampler() called an OpenGL method (glReadPixels) while not in a rendering pass. This somehow worked in the old Qt5/GLWidget implementation, because the current OpenGL context and FBO always remained set to the main view widget. But since switching to Qt6/QOpenGLWidget, this no longer holds true - the Qt6/QOpenGLWidget backend has a OpenGL compositor that works differently, and the OpenGL context/fbo of the main view was no longer current after the application was done refreshing.

Since we want to query the final rendered view using glReadPixels, we implemented a mu method called framebufferPixelValue(x, y) which internally eventually sets the OpenGL context/fbo to the correct one before calling glReadPixels. (after PR 707, there is no need to restore the previous one)

Describe the reason for the change.

See first paragraph of the "Summarize your change" section

Describe what you have tested and on which operating system.

I have tested the dropperSampler tool, with different layouts and docked windows, but on Linux64 only. Please test in Darwin and Windows.

Add a list of changes, and note any that might need special attention during the review.

Please test and review on all other platforms please; although I am confident this will work just fine. During investigating this bug, the easiest way to see this problem was to dock the annotation bar on the left portion of the application window, thus pushing the main image window a bit to the right. Then, you'd easily notice that the color was wrong when you picked a color, which was horizontally offset by the width of the docked panel.

The easiest way to test and repro this, and to notice the fix works, is to start TV with the "smptebars.movieproc" argument, where you can clearly see the differences in color.

If possible, provide screenshots.

n/a

[bernie-laberge]

Excellent.
Thank you @pbergeron-adsk !

[pbergeron-adsk]

Before merging, please merge PR 707, since there's a change in here that will only work if that PR is merged. I also need to add python bindings to frameBufferPixelValue()