continuing transition to multiple GL render modes
each render mode is a unique combination of:
- geometry construction (logic for converting GeoJSON source into GL geometries)
- GL vertex buffer layout
- GL program (vertex & fragment shaders)
testing styles & multiple render modes (added POI test layer)
simplify style helpers a bit by making them return functions (less function() { return ... } cruft in stylesheet)
saner default styles
skip rendering layers with no style defined
fix bug with UV texcoord generation for quads
continue testing multiple programs/render modes
(some hard-coded test code for alternate vertex buffer layouts - to be replaced/generalized)
don't leak z into global namespace
(TODO: proper module pattern for all files)
detect path that library was loaded from and adjust paths to allow other resources to be loaded (shaders, etc.)
bundle vector worker dependencies into vector worker package file, and load that directly as the worker (instead of calling importScript() to load dependencies)
Merge branch 'master' of https://github.com/bcamper/vector-map

Conflicts:
	dist/vector-map-worker.min.js
	dist/vector-map.debug.js
	dist/vector-map.debug.js.map
	dist/vector-map.min.js
	gl_renderer.js
don't set continuous animation for non-animated effects