Add case-insensitive filename searching for linux
Implement a simple image abstration and cache

Squashed from WIP/image-cache
Don't hardcode window height/width in cityview

Now we query the current window size, you can change the default size in
the settings.cfg and restarting
Add initial mouse selection

TODO:
- Go down z stack to find first non-empty (rather than just the current
  'maxZ' rendered)
- Change selection hit box? (Currently {0,0,TILE_WIDTH/2,TILE_HEIGHT/2}
  which maps to the top-left of each tile)
Merge remote-tracking branch 'origin/master' into WIP/win-deps
Fix windows build

Sync .cpp files with recent changes