Add/Subtract the projectable's offset whenever we go from coordinates
from our public interface (which are always image coordinates) and
coordinates in out internal tile pyramid (wihch always starts at 0,0).

The Gegl projection still needs an offset node that is missing.

Also, connect to the projectable's "update" signal using
g_signal_connect_closure_by_id() instead of simply g_signal_connect(),
so we really connect to the signal of the GimpProjectable interface
and not to the "update" signal of GimpDrawable (which is a conflict
that will happen on group layers).