using a render cache when the map is not being animated

when the map has no animation (for example for static datasets) when the user pans the map it feels pretty slow because torque needs to render all the points again

with this change the tiles are rendered and backed to an offscreen canvas so when the map moves torque does not need to render every point, just flip the image for every tile

Code to clean cache is also added, when the user zooms or a tile is loaded the caches are cleared
While compiler might optimize, move the "var tile" to first function line to avoid recreating it on each loop pass