iv wipe mode

There should be a mouse+modifier that splits the screen horizontally and/or vertically (depending on the mouse movement direction) of the current image and the previous image. (originally #292)

The issue identifies the iv viewer but does not name any files, tests, or entry points. Start by locating iv's mouse and modifier handling and how it displays the current and previous images. Done means a mouse-plus-modifier gesture splits those images horizontally or vertically according to movement direction.

Written by the indexing model from the issue text.