Based on viyiviyi zoom image extension.
a pure DOM manipulation for image zooming, dragging, and touch gestures on touchscreen devices
for txt2img and img2img image viewer.
what's different, the image will now always centered
and when its width/height exceeds the container's width/height
the image edges will always snap to the container's edges during dragging.
Usage:
Use mouse wheel/pinch to zoom in and out, hold left-click/touch to drag the image
