Method to check status of m_geometry in MapTile

hasGeometry returns false if no geometry vbos constructed for this tile.
Proxy Rendering

1. draw proxy tiles first
2. only draw when tile has vbos ready (hasGeometry returns true)
Managing proxy tiles

1. Fixed deletion of MapTiles from m_tileSet
2. Changed generation of proxy tiles:
    - any tile which is already loaded/vbo ready is passed to m_proxyTiles serially.
    - any tile which needs to call the tesselator (not vbo ready) is passed to m_proxyTile by the same async thread,
    started for visible tile fetching
3. Handles the case to set shared state, of the future corresponding to a non needed tile (not in the visible set now),
   to "nullptr"
4. More and better comments to explain whats happening.