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. 
Based on where it's set, `m_state` in `MapTile` is exactly synonymous with being in the set of visible tiles. Can we call it `isVisible` or something more specific than "state"?