add options for turning alpha, depth, culling on/off
(to be moved to a per-render-pass config in the future)
change depth test to be less-than-or-equal (less was causing incorrect z-ordering of alpha)

Conflicts:
	dist/tangram-worker.debug.js
	dist/tangram-worker.min.js
	dist/tangram.debug.js
	dist/tangram.min.js
	src/gl/gl_shaders.js
	src/scene.js