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Dynamic Paint: recursively search for island border edges.

It is quite likely in a triangulated mesh that the actual island edge
belongs to a different triangle than the current pixel; for example
consider corners of a triangulated axis aligned rectangle face that
have the additional edge: a pixel there will have to be assigned to
one of the triangles, but one of the edges of the original rectangle
can only be accessed through the other triangle.

Thus for robust operation it is necessary to do a recursive search.
The search is limited by requiring that it only goes through edges
that bring it closer to the target point, and also by depth as a
safeguard.

Differential Revision: https://developer.blender.org/D2409