Fix incorrect usage of the `flags` when drawing the preedit resulting
in setting the `flags`, but not actually reading the value back.

The logic to skip things was also used incorrectly, because the renderer
does that already based on the `WIDE_CHAR` flag on the cell.

Fixes: 67a433ceed (Skip whitespaces for wide chars in preedit)

Sextants are similar to quadrants and should align with them and
other box drawing, thus use builtin font to align them properly.

Part-of: #7422.

Fixes #7470.

With MSRV 1.70, std now contains the necessary parts.

Some hardware is just bad.

Fixes #7404. 

Iterate over points in line instead of drawing it right away
and then finding it in the buffer.

Fixes: 4a26667060 (Use builtin font to draw powerline symbols)

In addition to box drawing it was decided to also draw powerline
symbols, since those are quite common and rather simple to draw with
present box drawing infra.