Smokin' Guns support

Import q3map2 code by spoon from github.com/smokin-guns/GtkRadiant First adaptation GtkRadiant tree by Tequila, this adaption to NetRadiant by myself, some stuff is done so there is no need to build an SG-specific q3map2 binary.

Smokin'Guns use a .tex map sidecar to ship some custom surfaceflags to the game code. It seems to rely on some shader prefix to pick the right surfaceflag. The feature may be interesting for more games.