This patch adds a `confined` flag to each cell to track if the last
rendered glyph bled into it's right neighbor. To keep things simple,
bleeding into any other neighbor cell than the immediate right one is
not allowed. This should cover most use cases.
Before rendering a row we now do a prepass and mark all cells unclean
that are affected by a bleeding neighbor. If there are consecutive
bleeding cells, the whole group must be re-rendered even if only a
single cell has changed.
The patch also deprecates both old overflowing glyph options
*allow-overflowing-double-width-glyphs* and *pua-double-width* in favor
of a single new one named *overflowing-glyphs*.