Implement GL_EXT_draw_buffers.

Bug 19353282

Change-Id: I4a1782c2f1e9ae52b731ef447e97c353cc41044e
Reviewed-on: https://swiftshader-review.googlesource.com/5123
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
9 files changed