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
Guess is on purpose, but now there's no check that "this.isRunning() before rendering the tile (from cache or 1st time)