Reset the viewport when drawing with OffscreenSurfaces.

Fixes highlight and overdraw at
http://threejs.org/examples/webgl_multiple_views.html.
Merge pull request #494 from ScottTodd/viewports

Reset the viewport when drawing with OffscreenSurfaces.
Optimized mutator lookup, support list of mutators.
Merge pull request #496 from ScottTodd/optimize

Optimized mutator lookup, support list of mutators.