first commit of clipping mask in svg renderer
group.mask = poly works for svg and canvas renderers
extra check
confirmed group.mask = poly.clip and poly.mask = poly.clip
cleaning up
first commit of webgl clipping, unfinished
stencil buffer works, but is not helpful for this situation...
revert to state without stencil buffer
reorganizing API to be one way. Two.Group.clip and Two.Polygon.mask, not both inherited from Two.Shape
close but no cigar on webgl renderer
merged latest dev with clipping feature