color: NEVER EVER use memcpy() in critical path

Unless there is a very good reason to use memcpy(), don't use it.

Not using it makes the compiler not screw up and waste tons of CPU that
it could have not wasted.

Gets my framerate back from 1250 => 1750 and makes sysprof no longer
report ~40% of render time spent in gsk_gpu_colorize_op().