Implement trivial glInvalidate[Sub]Framebuffer.

Change-Id: I0790163f89116469f73ad9ff15c71dc7366a6f65
Reviewed-on: https://swiftshader-review.googlesource.com/4306
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>