According to the ES3 spec, there's no requirement for a
renderbuffer to be allocated until it is bound and any
non-zero renderbuffer bound must be allocated.

Change-Id: Id47083c7ec6e3b3698e176d6feff31121983e446
Reviewed-on: https://swiftshader-review.googlesource.com/4331Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>