Cont zoom

Tested rigorously specially after a crazy crash I found yesterday.

Summary of changes:

1. Uses non integral zoom
2. Arch specific code now handles pinch gesture continuously.
3. Managing Proxy Tiles:
  1. TileManager owns all proxy tiles (m_ProxyTiles)
    - proxy tiles are added serially when a previously loaded tile (in m_tileSet) has its vbos ready and asynclly when it needs to tesselate from already loaded map tile data.
    - proxy tiles are deleted once the corresponding visible tile finished loading.
  2. Update of tile sets takes care of:
    - Logical deletion of tiles from m_tileSet
    - Addition of new visible tiles to m_tileSet (and addition of corresponding proxy tiles)
    - Deletion of tiles (if not in proxy and logically deleted)

One small todo:
- Right now one gesture of zoom in and then zoom out without lifting fingers results in a zoom in first and then zoom out. So we have to lift fingers to start another gesture whenever we need to switch zoom-in/zoom-out. Will fix this once I figure out a solution for this. 
What if tiles just maintained a count of the other tiles that they serve as proxies for?

Say you zoom in on one tile [x y z] and the next zoom level becomes visible: Each new tile increments the count on its parent by 1. When a child finishes loading, it decrements the count on its parent by 1. Tiles can then be kept with the drawable tile set while their count is > 0 and safely removed when it reaches 0. 

Say you zoom out on a tile [x y z] and the previous zoom level becomes visible: Each new tile increments the count on each of its children by 1 (so [x y z]'s count is incremented by 1). When a new tile finishes loading, it decrements the count on each of its children by 1. Again, [x y z] can be kept in the drawable tile set until its count is 0 and then removed. 

In fact, I think we can safely do both of these operations for any new tile being loaded and then we don't need to care about whether we're zooming in or out. I think this could make the proxy tile management way simpler.