Improve Zooming and Panning in Lightbox

[krkeegan]

• Allow setting a minimum Zoom-Out scale. viewerMinZoom
• Allow resetting the view to the initial view when ZoomMin has been reached. This re-centers the image. viewerResetZoomAtMin
• Allow for RelativePinchtoZoom where successive pinching results in additional zooming rather than starting over each time. This functions much more like what people are used to on their phones. viewerRelativePinchZoom
• Define ZoomMax as where 1 pixel in the image equals 1 pixel on the display. The current version sets zoom max at 3 times the initial image size. On small screens this can mean very little zooming. On large screens this may allow zooming beyond 1:1.
• Allow defining a ZoomOut scaling factor for pinch to zoom. Zooming by default is slow because its scale is [0,1] where as zooming in is [1,inf]. A scaling factor of 3 seems to create the expected zoom out functionality. viewerZoomOutScale
• Limit panning so that the edge of the image is not allowed to go past the center of the viewer. Current version allows the image to be infinitely panned, which can cause user confusion.