ext.image_display: new _get_font_dimensions implementation (broken)

This allows drawing of images in gnome_terminal and tmux.  It is
currently broken, since the image is drawn at the wrong place.

Thanks to CasperVector, joehillen, majutsushi and progandy for the patch
and corrections.

See https://github.com/hut/ranger/issues/109 and
https://savannah.nongnu.org/bugs/?40888