improve dragmove performance

Is there any reason you want to fire MOUSEMOVE event (line 381, shape._fireAndBubble(MOUSEMOVE, evt); ) during dragging?

The this.hitCanvas.context._context.getImageData(pos.x, pos.y, 1, 1).data is very expensive during dragging. Dragging a shape feel choppy especially in a complex canvas with multiple layers. The proposed change improves the performance significantly (at least 10 times faster).