...support for transparency. Technically, the sixels do not have transparency,
but empty pixels are now rendered with the current background color instead
of black to make the them appear transparent. Same goes for the black bars.
The current background color makes them disappear.
There is one technical limitation with the alpha focus highlight patch.
The alpha value and background color is taken from the current background color,
so when the window is unfocused, images may have the wrong alpha and/or
background color. This can't be fixed easily.
Tested with and without alpha patch applied. Simply setting alpha to 255
seems to fix it. I didn't set `dst` on lines 263 and 273 because those
loops are impossible to reach.
