ioq3 compatibility libraries for gamelogic

[slipher]

We might want to add some optional libraries to src/shared/, for building into the gamelogic, to help porting gamelogic from other Q3-based engines. This would be code that is not used by Unvanquished, but would be useful to other games for adapting to differences in trap functions.

Legacy cvars: vmCvar_t

Resurrect the legacy gamelogic cvar library which was deleted in 12f42a0.

Model positioning/blending API

For Smokin' Guns I implemented trap_R_LerpTag with its ioq3 signature of trap_R_LerpTag( orientation_t *tag, clipHandle_t mod, int startFrame, int endFrame,float frac, const char *tagName ) using the entity cache APIs. (The one recently deleted from Daemon with skeleton changes had different arguments.) This is useful for porting MD3 model handling code. Also it would be nice to make the engine APIs more flexible, such that you could do tag queries without having to add an entity to the cache. Since I haven't tried to rewrite everything to use caching, for now it is necessary to add an entity to the cache, do the query, and then remove it.

[illwieckz]

Yes that would be nice. When I reworked some traps for optimization I was afraid of breaking things. Even though when keeping the unoptimized trap that is slow, at least it's compatible and can live right next.

[illwieckz]

Example of things I didn't merge was this:

• cg_api: implement trap_BatchGetUserCmds to fetch an array of command backups in one call #848

Since @slipher implemented a caching mechanism that allowed us to keep the legacy trap and avoid repeated calls:

• Keep a cache of usercmd_t's in cgame #854

Though we may still want to benefit from the reworked design that doesn't scatter (cached) calls everywhere, while keeping the compatible trap available.