Implement support for RGBA sampling.
Implement support for RGBA sampling.

Also improve the filtering precision of 3-byte formats.

Change-Id: If314c870735488dfcd22b9ab60b4783e31a4c05b
Reviewed-on: https://swiftshader-review.googlesource.com/2751
Reviewed-by: Nicolas Capens <0612353e1fcebbfd4b302d9337447ca0cdfa9122@google.com>
Tested-by: Nicolas Capens <0612353e1fcebbfd4b302d9337447ca0cdfa9122@google.com>

google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader