OpenLayers.Renderer.Canvas does not support vector dragging

[jscattergood]

The OpenLayers.Control.ModifyFeature uses the OpenLayers.Control.DragFeature class to allow the manipulation of vectors. This control does not work when the Canvas renderer is being used.

The root cause of the issue is that the canvas renderer's implementation of getFeatureIdFromEvent() only supports a single intersecting feature. In the case of the drag control, the drag handle overlaps the underlying feature. Due to the algorithm in getFeatureIdFromEvent(), it is finding the bottom vector feature and not the drag handle, thus the dragging is prohibited.

There a built in method for filtering overlapping geometries within the canvas renderer.

[fredj]

see #3207