Some types without alpha were not correctly setting the
alpha value, so copying to a type with alpha was causing
the image to not appear (due to it being almost completely
transparent).

Change-Id: Ib12c291bd5c51628e0579285731033372656f6a0
Reviewed-on: https://swiftshader-review.googlesource.com/4391Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>