Mac buildfix
Oops, forgot to reset the frame when unchanged.

Fixes #5861.
softgpu: Avoid multithreading thin polygons.

Small performance improvement (5-6% in some areas.)
Use the FLAG_EXECUTEONCHANGE flag.

Improves fps by 1-3% in some games, seems to be a win after all.
Don't check diff in ExecuteOpInternal().

Since it's not called if !diff, there's no need.  Small performance
improvement.
Merge pull request #5867 from unknownbrackets/gpu-minor

Avoid diff checks in ExecuteOpInternal, check in runloop
Merge remote-tracking branch 'origin/master' into master_0.9.8