Add CanvasGeometry

CanvasGeometry extrudes watertight solid meshes from pixels based on transparency(by default). It's based on a technique I'm researching and, so far, call running image triangulation & extrusion. There is still more to be done but it has already started being useful. The base concept comes from a blob tracking algorithm called run based component labeling. It works by recording continuous horizontal lines(separated by transparency) as pairs of vertices, called runs. While it raster scans one row, it interconnects the runs of the previous one, so most of the work gets done fast in one pass. It produces nice models with low polygon counts. You can see a demo here http://wizgrav.github.com/three.js/runner/