New intermission Hook for Y_Ticker

This branch adds a new hook that runs every tic during intermissions, allowing you to run any non-HUD related code during intermission. Works the same way as the ThinkFrame hook.

I don't have the time to check this but I believe this did not previously happen because of the Lua garbage collector not running on thus screen, and want to offload the doublechecking onto you

The Lua garbage collector is always running regardless of gamestate. According to @MonsterIestyn. lua_gc is called by LUA_Step, as long gL isn't null. And LUA_Step itself is called by D_SRB2Loop every frame.

merged

mentioned in commit 38770282