[css-properties-values-api] Resolve lengths for <image> values. (#780)

Values with <image> syntax can contain lengths, for example:
linear-gradient(red 1em, blue);

Author: andruud

css-properties-values-api/Overview.bs

@@ -417,7 +417,7 @@ For <url> values, the computed value is one of the following:
 
 For <image> values, the computed value is as specified, except that relative
 URLs that appear in the value are resolved to absolute URLs as described in
-[[!css3-values]].
+[[!css3-values]], and all lengths are resolved to their computed values.
 
 For <number> and <percentage> values which are not calc expressions, the
 computed value is as specified. Calc expressions that are