We don't yet take the spread-distance parameter into account, since we
don't have a way to "inflate" the text shadow.
Also, I'm not sure if we need to inflate the shadow slightly anyway.
Blurred shadows of our pixel fonts seem very faint. Part of this is
that a blur of < 3px does nothing, see #13231, but even so we might
want to inflate it a little.
The `text-shadow` property is almost identical to `box-shadow`:
> Values are interpreted as for box-shadow [CSS-BACKGROUNDS-3].
> (But note that the inset keyword are not allowed.)
So, let's use the same data structures and parsing code for both. :^)
We also pass whether the shadow goes inside or outside the element. Only
outer shadows are rendered currently, and inner ones may want to be
handled separately from them, as they will never interfere with each
other.
