[feature] configurable image offsets

[pirate486743186]

images are positioned assuming that w3m has the whole window of the terminal available. This is often not the case, for example they are menus bars, split terminals, tabs etc.... So often, images are missplaced.

It would be nice, if we could configure offsets for the position and size of the images, so that we can manually adjust the images to the terminal's real geometry.

This is relatively simple. It needs some extra configuration variables in the config file, that would be used by the function that calls w3mimgdisplay .