Clamp some filter functions to one if the values over 100%.

For grayscale, invert, opacity, and sepia, "values of amount over 100%
are allowed but UAs must clamp the values to 1" [1]-[4], so we clamp its value
in the parser.

[1] https://drafts.fxtf.org/filter-effects/#funcdef-filter-grayscale
[2] https://drafts.fxtf.org/filter-effects/#funcdef-filter-invert
[3] https://drafts.fxtf.org/filter-effects/#funcdef-filter-opacity
[4] https://drafts.fxtf.org/filter-effects/#funcdef-filter-sepia

components/style/values/specified/mod.rs

@@ -216,6 +216,15 @@ impl Number {
                                       -> Result<Number, ParseError<'i>> {
         parse_number_with_clamping_mode(context, input, AllowedNumericType::AtLeastOne)
     }
+
+    /// Clamp to 1.0 if the value is over 1.0.
+    #[inline]
+    pub fn clamp_to_one(self) -> Self {
+        Number {
+            value: self.value.min(1.),
+            calc_clamping_mode: self.calc_clamping_mode,
+        }
+    }
 }
 
 impl ToComputedValue for Number {