Blender Git Loki

Fix T83094: Alternate rows in the Sequencer are green (macOS)

The issue is that `UI_GetThemeColorBlendShade4fv()` creates a color
with alpha, but there is not any background underneath to blend in with.

The solution is just to draw an opaque background first, which also
halves the number of rects to draw. Note that the brighter rows get
very slightly darker after this change.

Differential Revision: https://developer.blender.org/D9947