Added both RGB8 and BGR8 formats to the blitter and related
functions so that these formats may be used with glReadPixels.

Change-Id: I22ee13f837b66af5f2135abc77fe81cc2e995fec
Reviewed-on: https://swiftshader-review.googlesource.com/4294Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>