Adding multiple color attachments to
Framebuffer objects. This cl allows
setting multiple draw buffers, but
there is currently no code exercising
that path. These will connected
properly to a shader's multiple
outputs later.

Change-Id: I41d4eadc4518d0b9c1468802c32e622df591e860
Reviewed-on: https://swiftshader-review.googlesource.com/2881Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>