Add additional fallback path for fixing XFB varyings on GL
Add additional fallback path for fixing XFB varyings on GL

* On AMD's driver at least the reflection for a separable vertex program
  includes all declared outputs even if they're statically not written in the
  vertex shader. However trying to use these as XFB varyings fails.
* Unfortunately this makes it impossible for us to use our typical reflection to
  determine what outputs to write, so instead we make a last ditch attempt to
  get things working and link the non-separable program we're about to use for
  XFB and reflect that again, then use the reflection to remove items from the
  varyings and replace them with gl_SkipComponentsN.

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