mf: Tighten bboxes of accidentals vertically

This series of commits improves the height and depth values of feta accidentals, replacing approximations with exact values. To do that, the code has been transformed to use 'sharpened' values (i.e., values not scaled to the output resolution) for bounding boxes. Fixes #6208 (closed).

While being there, also improve lower-resolution output of METAFONT. As an example, here are the renderings of feta11, glyph sharp (3 beams) at 300dpi; the middle beam in the old version has a different slope.

old:

new:

Finally, a small patch fixes issue #6209 (closed).