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The previous commit fixed flickering in the progress bar itself. Cursor
flickering is harder to address.

Cursor flickering could be fixed by hiding the cursor altogether while
the progress bar is displayed. The downside of this is that if the
program is killed in such a way that it can't clean up its state, it
would leave the cursor invisible.

Instead, this commit introduces an output buffer. All of the escape
codes and content for a single progress update are written to a buffer,
which is then flushed to the terminal all at once. This significantly
decreases the time during which the terminal has seen the cursor-hiding
code but has not yet seen the cursor-showing code, thus minimizing (but
not 100% eliminating) cursor flickering.

For more context, see:
https://gitlab.gnome.org/GNOME/vte/-/issues/2837#note_2269501

Previous code cleared the display before writing new content, creating a
window where the terminal could (and in some cases did) render empty lines.

Instead, we now write new content over the old content, only clearing
the trailing end of lines for cases where the new line is shorter.

Fixes #1664