transform: don't sample past-the-horizon points
When applying a perspective transform, don't sample past-the- horizon output points, which correspond to behind-the-camera input points, and should be clipped. Currently, we test each point individually, which is suboptimal. Ideally, we should precalculate the regions that are inside the transformed polygon, and only rasterize those. Either way, the output should now be the same.
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