Fix disallowing binding to the default framebuffer.

Bug 27166315

Change-Id: Ia9774e6560b326ca963830c53e030487c239164a
Reviewed-on: https://swiftshader-review.googlesource.com/4770
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>