* Introducing new Lua-exclusive function, P_IsValidSprite2(mo, spr2)....

* 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.
* All Lua-originated sprite2 settings are now forced through P_GetMobjSprite2. Makes sense because of SPR2_JUMP, which none of the main characters have sprites for yet all use.
* Cleaned up P_GetMobjSprite2 to not set irrelevant, otherwise-unused variable.