In 2.1 and 3.0, light_vec could be modified for altering shadow_computations.
But it broke shadows when rotating light. shadow_vec would do the same, but without breaking
shadows in rotated lights if not used.
Add inverse light transformation to shadow vec, so it's not affected when rotating lights;
Added usage define for shadow vec.
For shadow vec working properly when rotating a light, it's needed to multiply it by light_matrix normalized. Added usage define in order to don't do that if shadow_vec not used.
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