The "clear" operation can now be done through the blitter.
The few changes are:
- The blitter now supports RGBA masking
- The blitter now supports RGB565
- When in "clear" mode, the blitter does a one read/multiple writes

The old clearing code has been deleted from Surface.

Change-Id: I970c3a0323f63ee5c89f02d94a2705e4bcf83866
Reviewed-on: https://swiftshader-review.googlesource.com/4291Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>