In GimpOperationLayerMode, when the op has a mask connected, and
we're processing an area outside the mask bounds, set the op's
opacity to 0, so that the backdrop shows through.  The actual
process() function gets a NULL mask pointer in this case, and so
would composite the layer as if it had no mask, exposing areas that
should be masked out.

Add a GimpOperationLayerMode::parent_process() function, which
subclasses can override instead of GeglOperation::process(), and
make sure to update the GimpOperationLayerMode::opacity field
before calling this function (and, subsequently, before calling
GimpOperationLayerMode::process()).

Clean up the rest of the fields, and adjust the rest of the code.