This is the first op to obey the compositing color state. This means
from now on until all ops obey the ccs rendering is broken when ccs is
not set to linear.
I'll keep individual ops in seperate commits for easier review, because
they all need different adaptations.
Keeping the GdkRGBA requires doing later conversions, which isn't
necessary if we just keep the already converted float[4].
It also prepares for future color states, where the color will need to
be converted using the colorstate.
When drawing opaque color regions that are large enough, use
vkCmdClearAttachments()/glClear() instead of a shader. This speeds up
background rendering on particular on older GPUs.
See the commit messages of
bb2cd7225ece042f7ba10edd7547c1
for a further discussion of performance impacts.
