Fixes line rendering for wrapped lines in sdl2 frontend.

[mychris]

I am not sure if this is a complete fix for the rendering in the sdl2 frontend, but it fixes #1240
Maybe render-line should not fill to the end of the line but only to the width of the objects?

Or is it required that the objects given to render-line-with-caching always fill the whole line? If so, then redraw-logical-line-when-line-wrapping needs to be adapted to only call render-line-with-caching once per line.