Implement support for RGBA display formats.

Change-Id: I5d9c9567d6885a2c3bbaf78a821f66f99b1bfcfe
Reviewed-on: https://swiftshader-review.googlesource.com/2750
Reviewed-by: Ping-Hao Wu <pinghao@google.com>
Reviewed-by: Greg Hartman <ghartman@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>