split screen display : enables zoom bar tools of the input image

[pierrelabreche]

The zoom control buttons are currently disabled on the input image side when displaying "input" and "output" images in split screen.

Because mouse interaction occurs mostly on the input image, for example when using the speck removal tool, it would be ergonomic to enable the zoom bar controls on the input side. There would be less hand movement (more comfort) when using the mouse.