Add Silhouette class
Add Silhouette class

Silhouette emulates silhouette rendering. Performing tests like
RenderWebGL.pick can use Silhouette in place of reading a silhouette
rendering from the GPU.

Silhouette.isTouching(vec) takes a texture coordinate vector and checks
if the corresponding location is opaque or not for the underlying
information.

LLK/scratch-render,LLK/scratch-render