Added RG8 fragment output format

Added RG8 fragment output format

Tying up loose ends from the new output formats project.
One of the missing outputs was the RG8 format, which I added here.

Change-Id: I865ad4c462e892dbc3a7820003b2390b54453ee9
Reviewed-on: https://swiftshader-review.googlesource.com/7171
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>