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Ocean modifier: Expose eigenvectors for use as map

The eigenvectors in the ocean modifier (plus and minus) can be useful,
but are not exposed. Assuming the particle system was capable, the
eigenvectors could be used to drive spray emission velocities.

This exposes the controls to allow a map to be generated from these
eigenvectors. Currently, the values are mapped into a 0-255 range
similar to foam.

Differential Revision: https://developer.blender.org/D7182