Fix alpha type when doing GPU color space conversion of unpremul images

Fixes fringing seen on bottom row of
https://gold.skia.org/detail?test=makecolorspace&digest=eccdd2a952d9503734b35b3a725a18b2

Change-Id: I4d05a2edab99dc989f4088d38a13827384f0444a
Reviewed-on: https://skia-review.googlesource.com/150121
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed