[Pixman] RGB24 -> ARGB32 performance

[Tristan Schmelcher]

I have a Cairo application that is using Pixman to composite a
semantically opaque image by scaling a bunch of source images in RGB24
format. If the target surface is also RGB24 format, then it is very
fast and uses fast_composite_scaled_nearest_8888_8888_cover_SRC. But
if the target surface is the ARGB32 format, then it takes almost three
times as long and uses just fast_composite_scaled_nearest.

If I had a choice, I would always make the destination surface in
RGB24 format, but unfortunately it is being created as ARGB32 by
another library which I don't control, so my application is taking the
slow path. I want to improve it so that the performance in the RGB24
-> ARGB32 case is approximately as good as the RGB24 -> RGB24 case, if
possible.

My idea for how to go about this is to add a x888_8888 fast-path to
Pixman for RGB24 -> ARGB32 that does the same thing as the 8888_8888
path but sets all the destination alpha bytes to 0xFF. But before I
try coding it I wanted to ask here if anyone has any other suggestions
or knows of any "gotchas" with this approach.

Comments?

Tristan