Optimize R5G6B5 conversion.
Optimize R5G6B5 conversion.

Bug 20891368

Change-Id: I876e4dab9ab5ba7df5887d4b184e9750f9563edc
Reviewed-on: https://swiftshader-review.googlesource.com/3261
Reviewed-by: Alexis Hétu <1c7141772178a7e80858c3db5052b328076f08f4@google.com>
Reviewed-by: Nicolas Capens <0612353e1fcebbfd4b302d9337447ca0cdfa9122@google.com>
Tested-by: Nicolas Capens <0612353e1fcebbfd4b302d9337447ca0cdfa9122@google.com>

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