uniforms: map JS objects to GLSL structs
vertex layout: iterate over attribute values, not keys :/
uniforms: track texture units when setting GLSL structs
uniforms: support mapping arrays of structures
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
helper to get a tile XYZ for a given point in meters
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