We do a strip of triangles around the triangle of interest and modify the variying vars to "trick" the fragment shader into rendering what we need.
This keeps the compatibility for both shaders.
Vertex still get half displayed when angle at the vertex is very acute.
This patch is only for the simple, no vert clipped case.
This is 2.5x slower than without the trick (on my hardware with 1million tris).
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