[Bug] Fallback graphics are not displayed for SPR2_STIL

Sprite2 includes behavior that attempts to find fallback sprites if a graphics lump is completely unavailable. In the case of SPR2_STIL (sitting still and holding left), DRRR fails to fetch frames from SPR2_STIN, and produces console errors while displaying an error graphic. No such behavior happens for SPR2_STIR (sitting still and holding right), or for any other series of vanilla animations.

Given the nature of character creation for DRRR, most characters will never run into this issue, but SRB2Kart has a number of Simple AnimaL (custom animation lua) characters that are not designed with frames for turning left, and even my current demo character for Dr. Robotnik's Simpler AnimaL has a Lua patch to fix this hardcode bug.

The spr2defaults table does include an entry for SPR2_STIL to fallback to SPR2_STIN, but this doesn't actually work correctly because there is a condition in P_GetSkinSprite2 that is dependent on the current sprite not being SPR2_STIL.

https://git.do.srb2.org/KartKrew/RingRacers/-/blob/master/src/r_skins.c#L63

The equivalent code in SRB2 checks against SPR2_STND instead.

https://git.do.srb2.org/STJr/SRB2/-/blob/next/src/r_skins.c#L102

If this condition is fully necessary, then perhaps the condition should check against SPR2_STIN, since that appears to be the zeroth sprite2?