Only allow border-image-outset to use non-negative numbers.
Restricts border-image-outline to only allow positive values.
Fixes#14295
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This commit itself only moves things around and adds an extra parameter to the
`apply_declarations` function to eventually handle #14079 correctly.
Probably needs a more granular API to query fonts, á la nsFontMetrics, but
that's trivial to do once this is landed.
Then we should make the font provider mandatory, and implement the missing stylo
bits.
