zink: do not copy colors through floats

zink: do not copy colors through floats

Copying per compoents might flush NaN values, leading to changes in the
values, so it'd be safer to copy as unsigned integers here. But in one
of the cases here we can do even better, and just copy the whole damn
union instead.

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14932>