texture sprite/atlas support WIP
styles should re-initialize when updated w/new settings
texture icons WIP, pre-calc sprite UVs and use sprites for more render styles
uniforms: support mapping an array of textures
provide a built-in uniform array of textures
access parent style props through prototypes when cloning
more built-in support for textures/sprites
use isArray
convert color to vec4, use texture as color by default when present
convert point shaders color to vec4
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
turn off webgl backbuffer alpha for now to fix blending issues
(revisit, since this prevents us from having alpha 'underlays')
updated build/demo
sprites can specify a single size, or separate width + height
properly support size interpolation rules for sprites
remove redundancy in render loop
drop tesselator z option (not being used anyway and very small performance gain)
worker broker: add ability for worker to invoke a method in main thread (basically same API in reverse)
textures:
- handle single vs. multi-texture style syntax as a pre-process step in StyleManager
- better division of texture creation for Style in main thread vs. worker
(main thread needs to create shader uniforms and GL textures, worker only needs basic metadata for sprites)
textures:
- save a promise for texture loading, so other processes can await load
- provide a method to get (cloning/worker-safe) metadata about textures via promise
- use above method to sync texture info from main thread to worker