Retiming draw order changes in d0cf4a4a8b accidentally broke VSE
thumbnails. The order of drawing became "for each strip: draw thumbnail,
draw retiming continuity".
But the "draw retiming" bit was changing the projection matrix, which
placed all the later thumbnails at nonsensical locations.
Fixup it so that:
- First all thumbnails are drawn, then all retiming continuities.
- Retiming continuity drawing sets up matrix just once, instead of
doing all that work for each strip.
- And it also restores the previous projection matrix after it is done.
- Also, can_draw_retiming now says "false" when no retiming keys are
present. Previously, code was going into possibly expensive drawing
setup for any strip, even when the is nothing to draw.
Pull Request: https://projects.blender.org/blender/blender/pulls/124780
ce9becae4c