New entity: comp_pack_replace

[electrovoyage]

Probably not too useful, but I have some custom trigger textures that say the kind of trigger on them (e.g. for trigger_once there is a texture that says TRIGGER ONCE) and it would be really useful to replace these textures with tools/toolstrigger when compiling.

[TeamSpen210]

VBSP actually already has this feature, via the very poorly documented -materialsub option.