Add possibility to show tile numbers and render just one tile

Useful for debugging. Also, make the tile border drawing
optional. These three debugging features are governed by environment
variales (set in Xcode before running with Alt+Product>Run...)
DRAW_ONLY_TILE, DRAW_TILE_BORDERS and DRAW_TILE_NUMBERS.

Change-Id: I81f952284676eafe5d204c819658e0225aabdb1c
Bin a few since long commented-out lines

Change-Id: I4087f8f51dd6e8066818a98797c1e575ecca3442