* \param zfail action to take when stencil test passes, but depth test fails.
* \param zpass action to take when stencil test passes and the depth test
* passes (or depth testing is not enabled).
* \param zfail action to take when stencil test passes, but depth test fails.
* \param zpass action to take when stencil test passes and the depth test
* passes (or depth testing is not enabled).