Add enum for invalid/unused sprite id
Implement BlockMap sprite primitive

This use mainly usefuly for buildings which are build from a set
of 16x16 blocks
Corretly handle null sprites
Extend level file decoder
Add support for adding custom processors into level loading
Implement building rendering

* Make Renderer install its callback into LevelLoader to load required
  block maps; use these to actually paint buildings
Merge branch 'mapviewer'

Added necessary changes to map viewer to use newly added
Renderer/LevelLoader interaction functions
Add info on all gfx resources for first missing
Switch to seemingly more correct coordinate system