Change default pixel scaling method

[alexheretic]

The current PxScale method was inherited from rusttype and defines the pixel height of glyphs in pixels. There are different ways that users would expect this work.

My current thoughts:

• I don't want multiple px scale concepts inside ab-glyph. One is ok, and you should be able to convert that to any other.
• Changing how it's done would clearly be breaking.
• It would be nice to handle this in the least surprising & most common way as default.

Related: #11, #14

[TakWolf]

A possible reference:

96px in fontforge(left), chrome(center) and ab-glyph(right, scale_factor = px / font.units_per_em())

[LoganDark]

font-size should be in pixels-per-em. If users want something else they can do the calculations themselves from metrics in the head/os2 tables (not sure if ab_glyph exposes these)

for example, I use cap-height to size UI buttons. Though, I am not using ab_glyph because it does not support opentype features, but I am using ab_glyph_rasterizer which has a better API than anything else on crates.io