Get rif of unused counter.
Honour user priorities for the Avatar.

This was suppressed up to now because it caused NPCs to randomly
dissapear.  That was due to std::sort instabilities and is fixed by
assigning the NPCs a user render priority of 1 instead of 0.
Make sure NPCs are rendered each frame.

Make sure NPCs are rendered each frame by assigning a user render
priority of 1 instead of 0 when yielding the renderable.
Fix graphics caching in refresh() and resize().
Fix text rendering for non-SDL1 blit render devices.
Towards SDL2: don't refer to screen surface.

Get rid of remaining references to screen surface pointer in preparation
for SDL2 render device implementation.