Fix R5G6B5 blend.
Fix R5G6B5 blend.

blend() unpacks to ARGB order.

Bug 20891368

Change-Id: I9a505631ee0751d4ef4b01b82c8617f155e95ca5
Reviewed-on: https://swiftshader-review.googlesource.com/3280
Reviewed-by: Alexis Hétu <1c7141772178a7e80858c3db5052b328076f08f4@google.com>
Reviewed-by: Nicolas Capens <0612353e1fcebbfd4b302d9337447ca0cdfa9122@google.com>
Tested-by: Nicolas Capens <0612353e1fcebbfd4b302d9337447ca0cdfa9122@google.com>

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