Fixed dependency on undefined behaviour.
Fixed some minor issues in the physics module.

- Fixed errors in the Lua example.
- Refactored luaR_rawgetfield() macro.
- Refactored luaR_rawsetnilfield() macro.
- Renamed ptm_ratio -> g_ptm_ratio.
- Formatting.
Refactored luaR_rawsetfield() macro.
Refactored FontAtlas and improved label demo.