Reflect border_scale in outline cache keys

Reflect border_scale in outline cache keys

border_scale can change, e. g. when ass_render_frame is called twice with
the same renderer but different tracks. Glyphs with equal \bord tag values
but different border_scale values produce different border outlines and
hence should be distinguished in outline cache keys. To this end, store
scaled border widths (which are really used when generating the outlines)
in cache keys instead of \bord tag values.