On the Windows platform we get an initial bright white flash during
startup, before we start drawing on our canvas. This PR changes that
initial color to a mid-grey that is much less noticeable. After this
initial fill is done we then clear the background brush so Windows will
not use it later and slow down resizing by adding extra WM_PAINT
messages and fills.
Pull Request: https://projects.blender.org/blender/blender/pulls/137488
cf4778df74