Use pre-multiplied colors by default

Use pre-multiplied colors by default

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