Merge branch 'spr2stuff' into 'master'

Sprite2 changes

Some stuff!

* Lua access to sprite2.
    * Introducing new Lua-exclusive function, P_IsValidSprite2(mo, spr2). Basically just a wrapper for (((skin_t *)mobj->skin)->sprites[spr2].numframes > 0), useful for creating custom sprite2 defaulting functions since hooking into P_GetMobjSprite2 wouldn't be worth it.
* FF_ANIMATE support for sprite2s. The var2 of the state works identically to normal FF_ANIMATE, but var1 is completely disregarded - it just runs all of the frames available to that one sprite2 animation set.
    * As a result, a bunch of states which were either not previously animatable or had animated at constant speed now get animation without state changes.
* P_SetMobjState now supports sprite2 defaulting like P_SetPlayerMobjState does.

See merge request !51