ENH: Do not update pan until button release

[pllim]

Or something like that. From @astrofrog below:

Make it so that the image only gets updated when the user releases the mouse button, not during the panning. To me this would be the preferred solution - we only update during panning if the user has the button pressed but they don't move the mouse for a little while which is actually not super common behavior. So we could just update the image at the end of a pan event.

Follow up of spacetelescope/jdaviz#564 (comment) and spacetelescope/jdaviz#864 (comment)