Use generics for the filter property

This introduces an additional shadow type for drop-shadow().

components/style/values/generics/counters.rs

@@ -0,0 +1,51 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! Generic types for counters-related CSS values.
+
+use std::fmt;
+use style_traits::{HasViewportPercentage, ToCss};
+use values::CustomIdent;
+
+/// A generic value for the `counter-increment` property.
+///
+/// Keyword `none` is represented by an empty slice.
+#[derive(Clone, Debug, PartialEq, ToComputedValue)]
+pub struct CounterIncrement<Integer>(Box<[(CustomIdent, Integer)]);
+
+impl<I> CounterIncrement<I> {
+    /// Returns `none`.
+    #[inline]
+    pub fn none() -> Self {
+        CounterIncrement(vec![].into_boxed_slice())
+    }
+}
+
+impl<I> HasViewportPercentage for CounterIncrement<I> {
+    #[inline] fn has_viewport_percentage(&self) -> bool { false }
+}
+
+impl<I> ToCss for CounterIncrement<I>
+where
+    I: ToCss,
+{
+    #[inline]
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+    where
+        W: fmt::Write,
+    {
+        if self.0.is_empty() {
+            return dest.write_str("none");
+        }
+        for (&(ref name, ref value), i) in self.0.iter().enumerate() {
+            if i != 0 {
+                dest.write_str(" ")?;
+            }
+            name.to_css(dest)?;
+            dest.write_str(" ")?;
+            value.to_css(dest)?;
+        }
+        Ok(())
+    }
+}