Fix QMainWindow Resize Crash in Qt6

[williamwira]

Linked issues

Fixes #1063

Summarize your change.

On the GLView's first paintGL routine, the Rv Window's resizeToFit function is now called from inside a singleShot. This causes it to run after the first paintGL completes, which avoids the state that previously caused the crash.

Describe the reason for the change.

OpenRV, when built with the VFX2024 option which includes Qt6 6.5.3, had a crash regression. The crash would occur when resizing the main window after the preference "Fit Window to First Media Loaded" executed.

Describe what you have tested and on which operating system.

Tested on Linux RHEL 9.6. Made sure the steps below no longer resulted in a crash:

1. Build OpenRV with Qt 6.5.3 and the rvcfg flag -DRV_VFX_CY2024=1
2. Make sure RV > Preferences > Fit Window to First Media Loaded is active
3. From command line open RV with: rv colorchart,start=1,end=2,width=1920,height=804,fps=24.movieproc
4. After image loads, attempt to resize RV window. RV previously would crash, but should now should behave as expected.
5. In RV, activate menu option View > Lock Pixel Scale During Resize.
6. Close RV, then reopen with same command as in step 3. Image should load and resize to 1:1 scale.