This one can be used for both premultiplied and non-premultiplied alpha
formats, since alpha is always 255. It is useful for opaque PNG upload
on both cairo and GL renderers.
That way, all permutations are possible. Previously it was only useful
in the cairo renderer, which required rgba8 → premultiplied bgra8, while
the GL renderer required rgba8 → premultiplied rgba8. Now both are
available.
This was only useful when building for AArch32 without -mfpu=neon, on
AArch64 or with -mfpu=neon gcc is smart enough to do the auto-
vectorisation, leading to code almost as good as what I wrote in
1fdf5b7cf8.
This more than halves the total runtime of this function since the
previous commit, from 8.36% to 4.02%, and is most likely memory
bandwidth limited on this specific board now.
I tried to do a SSE2 version as well, but couldn’t find any equivalent
of the LD4/ST4 ARM instruction.
On x86 on a Kaby Lake CPU, this makes it go from 6.63% of the total
execution time (loading some PNGs using the cairo backend) down to
3.20%.
On ARM on a Cortex-A7, on the same workload, this makes it go from 57%
to 8.36%.
Make it use gdk_memory_texture_from_texture().
Also make gdk_memory_format_alpha() privately available so that we can
detect if an image contains an alpha channel.
Also, now make gdk_memory_convert() the only conversion functions
and allow conversions between any 2 formats by going via a float[4].
This could be optimized via fast-paths, but so far it isn't.
