Bopomofo glyph update suggestions

With the update of sample glyphs in Unicode, there are some glyphs now in Source Han Sans that could use a bit update:

1. U+312C BOPOMOFO LETTER GN ㄬ should probably have a vertical first stroke instead of slanted first stroke.
2. U+312E BOPOMOFO LETTER O WITH DOT ABOVE ㄮ should have a bottom similar to U+311B ㄛ.
3. U+31AD BOPOMOFO LETTER NGG ㆭ should have the rightmost leg lowered to baseline for balancing.

The current glyph in 2.004, reference glyphs from [I.Ming](https://github.com/ichitenfont/I.Ming) - reference for Traditional Chinese & bopomofo font, and [current glyphs in Unicode 15.0 code charts](https://www.unicode.org/charts/) are shown below.

Also, it would be better if U+3105 BOPOMOFO LETTER ㄅ and U+31A0 BOPOMOFO LETTER BU ㆠ can be expanded horizontally a little bit more; their width seems a bit small when in vertical typesetting.