Lua FOF flat names fix

Sequel to !263 (merged)

I apparently forgot to make ffloor_t's toppic and bottompic behave the same way as sector_t's floorpic and ceilingpic. Whoops!

Consistent with sector_bottompic, +1

Interesting that &levelflats[*ffloor->toppic]; replaces the old for loop, but it makes sense :)

Works as intended, and consistent too, so that's good! +1

merged

mentioned in commit c9a73034