Add 'sprite2' field to states

Backwards compatible:

• If state->sprite2 is 0, engine uses state->frame & FF_FRAMEMASK.

Backwards incompatible:

• Do not use FF_SPR2SUPER when setting actor->sprite2 or spr2defaults[]. Use SPR2F_SUPER or P_GetSuperSprite2(sprite2) instead.

New:

• Use P_GetStateSprite2 as a wrapper to get a state's sprite2.
• Use P_GetSprite2StateFrame as a wrapper to get a state's starting animation frame index.
• Use P_IsStateSprite2Super as a wrapper to check for SPR2F_SUPER or FF_SPR2SUPER.
• Use P_GetSuperSprite2 as a wrapper for (sprite2 & SPR2F_MASK) | SPR2F_SUPER.
• Limit raised from 68 to 964 (I think)