Fix 612

[smoelius]

Fixes #612

Note that I have to eat my words. This PR does not use the approach that @djc suggested:

I don't have a strong preference so open to arguments that I'm wrong, but I feel like a "line" being a horizontal stripe across the window (that is, counting multiple for wrapped lines) is probably the more intuitive concept?

I tried to do that, but I ran into trouble here:

indicatif/src/draw_target.rs

Line 390 in 8941d5e

orphaned.extend(self.state.lines.drain(..self.state.orphan_lines_count));

That is, DrawStateWrapper's drop handler uses state.orphan_lines_count as a number of state.lines entries, and I couldn't see a good way around that. So I adapted the other parts of the code to use the same approach.

This PR currently has five commits. I think they should make sense indiviudally.