Removes the need for a pre-allocated stack of fluid_preset_t's. Upon
adding a loader specific preset, a fluid_preset_t is automatically
created. defsfont and ramsfont now keep track of the fluid_preset_t's,
not the loader specific ones.
Also changes the sfont::iteration_next signature to directly return
the next preset. This is possible as presets are now only created once
and returned as a pointer.