The Corner Pin node as well as the Plane Track Deform nodes always
return a single color that appears to be the average of the input.
That's because the derivatives were computed in the sampler's space,
while they should be in texel space. Large derivatives meant that the
textureGrad function would always sample the lowest MIP level, hence the
constant average color.
This might be an issue with other uses of textureGrad in the compositor,
so their use should be investigated.
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