fix gl tile dealloc
remove a hard-coded color shade
add support for fractional zoom on GL map
(doesn't work well with leaflet panning, since pan is relative to integer zoom,
so pan speed changes based on integer zoom level)
move libtess.js triangulation helper functions to gl.js
remove unnecessary hasOwnProperty checks
fix setLineDash compatibility
use event.layerX/Y instead of offsetX/Y for better compability
more fiddling with fractional zoom
preserving previously visible zoom level above or below
(in future, should unload previous zoom tiles once new tiles are loaded)
minor cleanup
remove some commented test code
use pseudo-random color based on geometry id for buildings, to keep color consistent between zoom levels