Multiple color attachments

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/2881
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
9 files changed