[CURVE/RUNE]: WebGL context loss not handled

[Cloud7050]

When the canvas used by the curve or rune modules loses its WebGL context, the canvas stops functioning and the module does not recover. Because the webglcontextlost event is not handled and does not preventDefault(), the browser does not eventually restore the context (thereby triggering the corresponding webglcontextrestored event to allow the module the chance to recover).

The modules should be updated to handle this scenario similar to CSG (see #146).