High res tiles

Refers #172 Now 

1) We render tiles when all of them are done loading
2) We figure out what tiles are visible and hide or delete only the ones that are not visible

My initial feeling was that it might be useful for some layer / code to query the last zoom map was set to. Initially I was using it for the osmLayer, however for it, using the timer id was more useful since that would be different even when the zoom level remains the same (when panning for instance). I am willing to take this out and the m_lastZoom if we think that it won't be that useful ( I am in favor of removing it bit more ).