Grabbed windows don't update content when not being moved

[git-f0x]

If grabbing a window with changing content (e.g. a video), the content is updated while the window is being moved. When the window is grabbed but not being moved, the content stays frozen at the last frame.
Easy to test with vkcube.

This doesn't happen if something is capturing the screen (e.g. OBS), so it can't be recorded.

[git-f0x]

Seems to be fixed (likely by #1457)!