Only process clip/cull distances if they're actually outputted from the vertex shader.

There's still overhead of having these distances stored in each vertex / primitive, but we should optimize the structure sizes as a larger set of changes.

Bug: b/139207336
Tests: dEQP-VK.clipping.*
Change-Id: I8f04b1c3ea823bb1a8cf62f18c987e01cd0c979a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/35032Reviewed-by: Chris Forbes <chrisforbes@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>