wlr_scene_rects expect their color to be pre-multiplied
while cairo_set_source_rgba() expects them to not be
pre-multiplied. With this patch we now use premultiplied
colors internally by default and then reverse it when
setting cairo colors.
This ensures the titlebar uses a consistent color in case
it was defined with some transparency by the user.
Fixes: #1684
...and unify region overlay and snap-to-edge overlay into overlay.c.
Snap-to-edge overlay is delayed for 500ms to prevent flickering when
the view is dragged from an output to another (demo in discussion labwc#1613).
This also fixes a bug that region overlay is not shown when a modifier
key is re-pressed.
