Normally the vertex processor will clip geometry that's outside of the
near and far planes. However, when depthClamp is enabled, depth clipping
must be disabled. So this change also enables the extension that allows
users to explicitly control depth clipping.

Bug: b/185814882
Tests: dEQP-VK.clipping.clip_volume.depth_clip.*
Change-Id: Iaab31c17cac382cf55a8c50d8e7e7d4c87b55272
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/53968
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Commit-Queue: Sean Risser <srisser@google.com>
Tested-by: Sean Risser <srisser@google.com>