Implement glReadPixels() for RGBA formats. Change-Id: Ia2f7d1db3b2a0f075326c3331125ca347f42932d Reviewed-on: https://swiftshader-review.googlesource.com/2756 Reviewed-by: Nicolas Capens <capn@google.com> Tested-by: Nicolas Capens <capn@google.com>