The 'avar' table does not allow random access to axis maps,
so change API to avoid quadratic-time implementation.

Removed -hb_ot_var_normalize_axis_value(), added
+hb_ot_var_normalize_variations() and
+hb_ot_var_normalize_coords() instead.

Hopefully also fixes build failure on msvc.

hb-view correctly renders variations with ft font-funcs now.
hb-ot-font needs HVAR implementation.

Is hooked up to the font, but not to FreeType, so raster doesn't show yet.

Documentation needs to be done.

Looks much better.

Supports enumerating variation axes, normalizing values, etc.

The numbers for right-to-left scripts are processed also from right to
left, so the order of applying “numr” and “dnom” features should be
reversed in such case.

Fixes https://github.com/behdad/harfbuzz/issues/395

Resolves https://github.com/behdad/harfbuzz/issues/324