mesa: introduce a clear color union to be used for int/unsigned buffers

mesa: introduce a clear color union to be used for int/unsigned buffers

This introduces a new gl_color_union union and moves the current
ClearColorUnclamped to use it, it removes current ClearColor completely and
renames CCU to CC, then all drivers are modified to expected unclamped floats instead.

also fixes st to use translated color in one place it wasn't.

Signed-off-by: Dave Airlie <airlied@redhat.com>