Fetches a top-accent-attachment value (if one exists) for the specified glyph index.
For any glyph that does not have a top-accent-attachment value - that is, a glyph not covered by the `MathTopAccentAttachment` table (or,
when font has no `MathTopAccentAttachment` table or no `MATH` table, any glyph) - the function synthesizes a value,
returning the position at one-half the glyph's advance width.
| font |
Font to work upon |
| glyph |
The glyph index from which to retrieve the value |
|
the top accent attachment of the glyph or 0.5 * the advance width of |