Clear images without full redraw + allow w3mimgdisplay options

According to w3m documentation, passing options to w3mimgdisplay may be required in some specific cases.
For example, an unknown terminal emulator may require to change display offset.
W3mimgdisplay draws a rectangle of the background color over the display area to clear it. Therefore, it needs to know which background color is used. The background color of X terminal emulators is detected automatically but not Linux frame buffer one (it defaults to black).