+ things as sprites on map, controlled with am_showthingsprites CVAR

drawing of sprites instead of things/players markers with am_cheat >= 2
possible am_showthingsprites values are:
0 - sprites disabled
1 - static front sprites
2 - animated front sprites
3 - animated sprites with actual things angles

![screenshot_doom_20130725_170602](https://f.cloud.github.com/assets/777125/855968/85bdace4-f533-11e2-98c6-5938383ca24d.png)
![screenshot_doom_20130725_170639](https://f.cloud.github.com/assets/777125/855969/89d746e6-f533-11e2-8c18-30dc40a1f000.png)