This patches rewrites the GPU shaders of curve nodes for easier future
development. This is a non-functional change. The new code avoids code
duplication by moving common code into BKE curve mapping functions. It
also avoids ambiguous data embedding into the gradient vectors that are
passed to vectors and reduces the size of uniforms uploaded to the
shader by avoiding redundancies.
This is needed in preparation for the viewport compositor, which will
utilize and extend this implementation.
Reviewed By: fclem
Differential Revision: https://developer.blender.org/D14689
