Autohinter support for box drawing unicode range

Migrated from: [SAVANNAH-52215]

For the box drawing to work, the characters in this range (U+2500-U+257F) have to be monospaced and span the exact distance between the descender and ascender lines. This distance and the distance between the lines is always rounded to pixels. Therefore, arbitrary scaling does not work for these characters. We should either

1) use discrete scaling that corresponds to the rounded metrics, or
2) round the edge pixels to the ascender and descender lines in the autohinter.

Note that, it is probably incorrect to modify the ascender and descender lines and line spacing for these characters because they are routinely used with other normal glyphs to draw tables.