babl: add babl_gc

The LUT garbage collection was running ad-hoc as a side effect of processing,
this is problematic since it could trigger during processing of other threads
that might be using or freeing the same fish leading to races. The new babl_gc
function is to be called from the main thread, at times when processing is not
occuring, calling babl_gc is not neccesary in short lived commandline tools
and similar.