GL_SRC_ALPHA_SATURATE can now be used as
a dst type.

Change-Id: I48764a8fb8900d418de3180cf1a485871582e931
Reviewed-on: https://swiftshader-review.googlesource.com/3104Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>