Vulkan doesn't have user clip planes in the legacy GL sense -- there is
an optional feature to supply GLSL1.30-style clip distances out of the
vertex shader, but the actual plane handling is the shader's problem.

We also don't support this optional feature.

Bug: b/125909515
Change-Id: I6dfc3eda613982112786a52517d1cd126146c590
Reviewed-on: https://swiftshader-review.googlesource.com/c/25329Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>