Some checks in Framebuffer related functions were still
ES2 only. Made some modification to fix the checks for ES3.

Change-Id: Iaef6133179ea830dd599a5d66bc9698032085569
Reviewed-on: https://swiftshader-review.googlesource.com/3690Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>