The code was mixing logical and physical sizes, causing glyphs being
doubly scaled down. Instead, this patch introduces:
* Physical* and Logical* euclid length/size/rect aliases
* some extraction traits for getting the scalars in rects/sizes as lengths (until euclid has them
built-in)
* wrapper traits/types for safely extracting the physical font metrics the
compiler generates (i16)
* Fix a bug in the text height calculation where we failed to take the
descent into account
Enable with `SLINT_EMBED_GLYPHS=1` and select sizes like
`SLINT_FONT_SIZES=12,16`
This change just puts the data structures in place, rasterizes a fixed
subset, embeds that into the rust generated code and calls a backend
function for registering the font that is unimplemented.
