We have a limited number of interpolants, and 2 were used for spotlights,
but this is a fairly uncommon case, so we move the calculation to the
fragment shader. A side effect of this is that we are not limited to
two spot-light anymore from the shader's point of view.
This essentially adds a vector transform per fragment, but on the other
hand, it removes communication between the vertex and fragment shader,
as well as interpolation of the position.
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