Render targets with only a red or alpha component were getting swapped
values within pixel quads at the edge of primitives. Also provide
format mappings for GL_EXT_texture_rg.

Change-Id: Ifb395039fb2958400ab36d5d184d7952d60fa9c6
Reviewed-on: https://swiftshader-review.googlesource.com/13268Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>