NOTE: This change shouldn't have any visible effect. It's just the
first (easiest) step towards decoupling gizmo redraws from viewport
redraws.
We currently redraw the entire region whenever a gizmo needs redrawing,
which would be nice to avoid in the future, see T73198. The first step
towards this would be having a separate tag for them, which is what
this patch implements.
The term "editor-overlays" was chosen because for the forseeable future,
we'll also have to redraw non-gizmo overlays in-between drawing 3D and
2D gizmos. Namely annotations.
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D6838
c4b9cb0af0