Measure the single advance width with a heuristic method

This fix is needed to use dual-width fonts, which have double-width
glyphs (e.g. CJK unified ideographs).

Signed-off-by: Ryusei Yamaguchi <mandel59@gmail.com>
Signed-off-by: Christoph Lohmann <20h@r-36.net>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea and freedom
500 Internal Server Error

Gitea Version: 1.28.0+dev-505-g31b4d79a84