mesa: Skip clearing color buffers when color writes are disabled.
WebGL Aquarium in Chrome 24 actually hits this.
v2: Move to core Mesa (wisely suggested by Ian); only consider
components which actually exist.
v3: Use _mesa_format_has_color_component to determine whether components
actually exist, fixing alpha format handling.
v4: Add a comment, as requested by Brian. No actual code changes.
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Tested-by: Dylan Baker <baker.dylan.c@gmail.com>