Previously, when overwriting wide graphemes, those were not first
cleared from the buffer. This could sometimes result in partial
graphemes, which is an invalid buffer state.
Now, when a wide grapheme is overwritten, it is removed from the buffer
entirely and replaced with spaces. Those spaces retain the style of the
removed grapheme.
A similar thing now happens with graphemes that only partially overlap
the buffer, either on the left or the right border. Those parts that are
inside the buffer are rendered as spaces with the grapheme's style.
