Implement R5G6B5 clear. Bug 20891368 Change-Id: I5c283589c1fb08003194e668c66ade4d0fe36842 Reviewed-on: https://swiftshader-review.googlesource.com/3250 Reviewed-by: Nicolas Capens <capn@google.com> Tested-by: Nicolas Capens <capn@google.com>