Tile polygon clipping is inefficient

[gordonwoodhull]

Related to #3 flag for controlling tile polygon clipping, the implementation of tile polygon clipping is inefficient because a clipping path is created for every tile.

There only needs to be a clipping path for each base polygon, because the clip path is relative to the tile center point.

The easy way to implement this is to put the SVG clip paths into a hash and assign an ID. However, for clarity and for debugging purposes, it would be better for polygons to identify their base polygon name/ID, and use that to id their clipPath and the association with tiles.

Then there also the cube grid case, where only one clip path is needed because the square is transformed three ways.