integrate XGM into the code

New turf proc: assume_gas().  Maps to air.adjust_gas_temp().
Lots of optimizations to processing, fire, lighting, HasEntered() and
 more.
Zones now process all fire data and existance in one go, fire objects
 only handle spreading.
Most code has been ported straight so some of it mightn't be ideally
 structured for the new gas_mixtures.

Signed-off-by: Mloc-Argent <colmohici@gmail.com>
make gas mask filtering more generic

Signed-off-by: Mloc-Argent <colmohici@gmail.com>