/explore map overlay coordinate conversions cleanup

[dognotdog]

The map overlay has a bunch of coordinate conversions functions between pixels and CSS/canvas pixels, and convenience functions that support the operations needed for UI and display.

These have some inconsistency in terms of using CSS/canvas coordinates, and wether google.maps.LatLng objects are accepted and returned or simple {lat,lng} objects.

It seems like it would be a good idea to

• remove to/from canvas conversions, as WebGL coordinates are setup to represent CSS pixels not device pixels
• make coordinate transform functions universally return {lat,lng} objects, but accept google.maps.LatLng as well for input parameters.