Use matching internal and external formats for RGBA8 images.

Change-Id: I056e1a4c7ace83207bed85ce28aca187d72adc6f
Reviewed-on: https://swiftshader-review.googlesource.com/2757
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>