Correctly return zero vertices if clipping plane 0 or 2 clip away the

entire polygon.

This fixes a bug that was introduced three years ago in BZ bug 1268854.
What happened was that the final pass over the polygon assumed that the
current polygon was living in plane[0]. But due to the double buffering,
the "current" polygon alternates between plane[0] and plane[1].
The bug had also introduced an early exit so that we could hit the final
pass at a time where the current, now empty, polygon was in plane[1]. So
we would incorrectly treat all 32 points in plane[0] as part of the
final polygon.

This bug was responsible for intermittently unreasonable numbers in
CompositorOGL's fill rate / overdraw overlay.

This fixes a regression caused by the fix for CVE-2016-5252.

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