For now for crop, linear-gradient and radial-gradient, lua files are live
searched at runtime each time the ui is redrawn, this permits live editing of a
copy of gegl_crop.lua or any new introduced per-op ui in /tmp or
/home/user/.local/share/gegl-0.4/lua/gegl_crop.lua to override the system wide
on installed in /prefix/share/gegl-0.4/lua/gegl_opname.lua and when running the
gegl binary out of the build, the in-build lua files take precedence.

The following are available as globals for the lua scripts:

loaded lua modules: ffi Mrg lgi
lgi namespaces: GLib Gegl GObject

mrg    mrg context for registering interaction callbacks and rendering xhtml/css
cr     cairo context, transformed to the local coordinatesystem of the op
active active GeglNode
sink   the sink GeglNode
dim    a dimension suitable for a touch interaction area (for now 10% of mrg:height())
o      (the global application state struct)