AFAICT, css-values-4 tells us that clamping numbers to a range where
min>max is okay. That means we can't use AK::clamp() since it will
VERIFY that max>=min.
This patch adds a css_clamp() helper (locally in FFC for now).
This fixes an issue where a bunch of sites were crashing due to the
VERIFY in AK::clamp().
cc77c82383