The previous fix wasn't handling the case where the same fragment
output variable would go through declareFragmentOutput() multiple
times, so this cl fixes it.

Fixes all dEQP-GLES3.functional.shaders.random failures.
Still fixes original WebGL2 conformance tests failures in:
all/deqp/functional/gles3/fragmentoutput

Change-Id: Ia9c4f5ed5c444ab0c020cac8be511fcaad23c55d
Reviewed-on: https://swiftshader-review.googlesource.com/16928Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>