Fixes line-rendering for wrapping and scrolling.

[mychris]

The next attempt to fix line rendering for #1243

The sdl2 front-end requires the whole line to be rendered. Otherwise, the caching of lines won't work. It also doesn't like empty strings.

The left-side-width depends on the front-end. For ncurses, it is the number of characters, for sdl2 the number of pixels.

I also tried to fix the scrolling version with line-numbers. Because the line-numbers are appended before the calculations are done, those will be scrolled out as well.

Hopefully this works now.

[mychris]

I added another change which makes use of the window-left-width in the calculations of virtual lines. This makes the navigation of next-line and other functions better, when line-wrapping and line-numbers are enabled.
It is not perfect yet, but seems better than before.

[cxxxr]

Thank you