simple global vertex fog support + some bug fixes

- added simple global vertex fog (uses standard 3DGS fog variables
view.fog_start, view.fog_end, d3d_fogcolor1, etc...)
- rewrote the wrapper functions. After level loading call
sc_create(mode) (mode can be SHADEC_LOW, SHADEC_MEDIUM, SHADEC_HIGH,
SHADEC_ULTRA), these predefined modes can be edited in sc_wrapper.c in
function sc_get_settings().
- fixed memory leaks
- added alpha clipping to PSSM sun shadow
- updated the example levels