Windows 11 support for DISCARD swap effect seems to be buggy and slow.
Use FLIP_DISCARD instead. Staying with DISCARD on Windows 10 because of
reports of flickering that hopefully won't happen on Windows 11.
We're also not using ALLOW_TEARING because it seems to break after an
OBS cycle of minimize/restore on both Windows 10 and 11. The swap chain
displays a stale image. Not sure if ALLOW_TEARING would be beneficial
even if it was working.