Skip byte->float conversion if output file format
supports high bit depths but configured to only
output 8 bits per channel.
Gives around 30% speedup when re-exporting movie file
to PNG image sequence here on laptop.
Possible further optimization:
- Skip color space conversion in imbuf_for_write
function if we've got already have buffer in
that space.
This doesn't seem to happen often after tweak
to render result to imbuf.
- Skip black alpha-under if original image is
opaque,
This is a bit tricky to detect.
885354daec