By moving the application of view_alpha after pre-multiplication we can
simplify main() considerably.
The cost is that for straight-alpha input or color_pipeline() we might
be doing three multiplications more than before. However,
a) the cost of running color_pipeline() probably dominates anyway, and
b) to get straight-alpha input you have to use a future Wayland
extension that probably won't be advertised without color management.
So we keep the optimization for the simple case (no color management)
while potentially incurring a small cost on the heavy case (with color
management).
Thanks to Pierre-Yves Mordred for the inspiration in
https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/889#note_1411774
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>
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