Simplified proxy tile logic

Simplied tileManager's logic to handle proxy tiles
- m_tileSet contains visible as well as proxy tiles
- every tile maintains a counter which tells for how many tiles this tile is acting a proxy
- cleaned a bunch of code around proxy tile handling

NOTE: Right now to avoid, mutex around setting m_tileSet, I am not doing proxy tile addition for which tesselation needs
to be done. Will update the code to handle this. However would like to have some comments whether this should be done or
not!