add (commented out) preserveDrawingBuffer flag,
which is needed for gl.readPixels, and could be used for feature selection in the future
increase max zoom to 20
works well for webgl, but have seen zooms > 18 cause seams in canvas, so may want to vary per-renderer
whitespace
add Seattle to common locations
explicitly set tile div size, which ensures tiles will receieve events for onmousemouse, etc.
move z-coordinate normalization to vertex shader (out of GL geometry construction)
add (commented out) test code for reading WebGL surface color on mouse-over
could be used in future for feature selection
(using same technique as canvas renderer, rendering to separate offscreen collision-map surface)
move tile layers into tile.layers to avoid conflicting with other properties
added (commented out) rotation test in vertex shader, may use for isometric-style projection in the future
cleanup some unused shader uniforms
add '3d' building effect (non-perspective, ground polygon extruded upwards)
add culling for back-facing polygons