Make the default distance calculation distance = 0

As far as I can tell, this produces the correct result in all cases.

This closes w3c#156.

Author: birtles

Overview.bs

@@ -3833,7 +3833,7 @@ type.
 For <a>animation types</a> that do not define a specific procedure
 for <a>distance computation</a> or which are defined as <dfn>not
 paceable</dfn>, the <a>distance computation</a> procedure is simply
-<var>distance</var> = 1.
+<var>distance</var> = 0.
 
 <h4 id="not-animatable-section">Not animatable</h4>
 
@@ -3941,7 +3941,7 @@ calc</dfn> has the following behavior:
     When there is insufficient context to calculate a used value for all
     possible values of <var>V</var><sub>start</sub> and
     <var>V</var><sub>end</sub>, the result of the <a>distance computation</a>
-    calculation is 1 (i.e. the behavior degenerates to <a>not paceable</a>).
+    calculation is zero (i.e. the behavior degenerates to <a>not paceable</a>).
 
     <div class="example">
       For example, for a <a>keyframe effect</a> that does not have