Enhencement : being able to call a function while dragging

[pierat]

Should be interesting to have an event enabling to refuse/accept the dragging horizontal and/or vertical user action.

Use case : I use the plugin for dragging a zoomed image within a viewport and I would like to refuse the dragging out of the container. This is to say top and left css attribute should be, for example, comprise within the [-15,+15] interval.