Incremental Style Recalc

This patch puts in the initial framework for incremental reflow. Nodes' styles
are no longer recalculated unless the node has changed.

I've been hacking on the general problem of incremental reflow for the past
couple weeks, and I've yet to get a full implementation that actually passes all
the reftests + wikipedia + cnn. Therefore, I'm going to try to land the different
parts of it one by one.

This patch only does incremental style recalc, without incremental flow
construction, inline-size bubbling, reflow, or display lists. Those will be coming
in that order as I finish them.

At least with this strategy, I can land a working version of incremental reflow,
even if not yet complete.

r? @pcwalton

components/layout/incremental.rs

@@ -2,6 +2,8 @@
  * 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/. */
 
+use std::fmt;
+use std::sync::Arc;
 use style::ComputedValues;
 
 bitflags! {
@@ -35,6 +37,32 @@ impl RestyleDamage {
     }
 }
 
+impl fmt::Show for RestyleDamage {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::FormatError> {
+        let mut first_elem = true;
+
+        let to_iter =
+            [ (Repaint,      "Repaint")
+            , (BubbleISizes, "BubbleISizes")
+            , (Reflow,       "Reflow")
+            ];
+
+        for &(damage, damage_str) in to_iter.iter() {
+            if self.contains(damage) {
+                if !first_elem { try!(write!(f, " | ")); }
+                try!(write!(f, "{}", damage_str));
+                first_elem = false;
+            }
+        }
+
+        if first_elem {
+            try!(write!(f, "NoDamage"));
+        }
+
+        Ok(())
+    }
+}
+
 // NB: We need the braces inside the RHS due to Rust #8012.  This particular
 // version of this macro might be safe anyway, but we want to avoid silent
 // breakage on modifications.
@@ -47,7 +75,13 @@ macro_rules! add_if_not_equal(
     })
 )
 
-pub fn compute_damage(old: &ComputedValues, new: &ComputedValues) -> RestyleDamage {
+pub fn compute_damage(old: &Option<Arc<ComputedValues>>, new: &ComputedValues) -> RestyleDamage {
+    let old: &ComputedValues =
+        match old.as_ref() {
+            None => return Repaint | BubbleISizes | Reflow,
+            Some(cv) => &**cv,
+        };
+
     let mut damage = RestyleDamage::empty();
 
     // This checks every CSS property, as enumerated in