Moved drawing line code from drawing_context back to screen.

Moved drawing line code from drawing_context back to screen.
The SDL code has the following issues:
- it writes right away to the screen. Not waiting for the updating screen call.
- it is extremly slow, since our screen is a hardware surface.

Would be neat to have a draw_line func in drawing_context, but looks like it will have to wait.

SVN-Revision: 1513