MASSIVE speedup when rendering thousands of lines at the terminals bottom.

This speeds up the output of a typical `find` command (with ~10k lines
printed to the terminal) by a factor of 10 or more.
Fix terminal resizing.
Restore cursor when DECSAVECUR mode is reset.
Do not call draw_string when there is nothing to draw.

In this case, the chunk starts with an escape sequence or another control
character.