If a viewport is outside maximum bounds of the render buffer,
then there's nothing to render, unless transform feedback is active.
If transform feedback is active, then we zero out the viewport.

This is to prevent an overflow in viewport-dependent calculations.

Bug chromium:969353

Change-Id: Ibc65447ca2b46e942b9934304a56b90f261e2bf1
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33068Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Sean Risser <srisser@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>