VR: Fix big performance bottleneck for simple scenes

Blender's main loop puts the main thread to sleep for 5ms if no user input was
received from the OS. We never want that to happen while the VR session is
running, which runs on the main thread too.

For simpler scenes, where the viewport already draws fast, this may have quite
some impact. E.g. in my tests, the classroom scene went from ~55 to quite
stable 90 FPS in solid mode (total render time as measured and averaged by
Windows Mixed Reality utilities). With Eevee, it only went from 41 to 47 FPS.
In complex files, there's barely a difference. E.g. less than 1 FPS increase in
a Spring file (both Solid mode and Eevee).

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