UMap:getControlInput should return a valid control input
experimental: allow early prepare for FuncUMapDef

this could be used in case for example Udef.shouldPlayOnFunc depends on
the outcome of the function, and shouldn't change after that. In that
case use uPrepareValue on it (can be called upon any value, but only
does something on FuncUMapDefs)
add DisplayPointSpec

used to only display a point