[css-easing-2] Make linear() accept the before flag, like stepped functions, with the same behavior for discontinuities.

Author: tabatkins

css-easing-2/Overview.bs

@@ -294,7 +294,8 @@ Output</h4>
 
   To <dfn export>calculate linear easing output progress</dfn>
   for a given [=linear easing function=] |func|,
-  and an [=input progress value=] |inputProgress|,
+  an [=input progress value=] |inputProgress|,
+  and an optional [=before flag=] (defaulting to false),
   perform the following.
   It returns an [=output progress value=].
 
@@ -305,8 +306,12 @@ Output</h4>
     of that item.
 
   3. If at least one of |points| has an [=input progress value=]
-    matching |inputProgress|,
-    return the [=output progress value=] of the last such point.
+    matching |inputProgress|:
+
+    * If the [=before flag=] is true,
+      return the [=output progress value=] of the first such point.
+    * Otherwise,
+      return the [=output progress value=] of the last such point.
 
   4. Otherwise, find two [=linear()/control points=],
     |A| and |B|,