[css-transforms-2] Add ''perspective(none)''.

This adds the ''none'' argument to the ''perspective()'' function,
equivalent to infinity.

Fixes w3c#6488.

Author: dbaron

css-transforms-2/Overview.bs

@@ -122,7 +122,7 @@ Terminology {#terminology}
 	''rotateY(0)'',
 	''rotateZ(0)''
 	and ''matrix3d(1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1)''.
-	A special case is perspective: ''perspective(infinity)''.
+	A special case is perspective: ''perspective(none)''.
 	The value of m<sub>34</sub> becomes infinitesimal small
 	and the transform function is therefore assumed to be equal to the identity matrix.
 
@@ -983,7 +983,7 @@ In the following <dfn export>3d transform functions</dfn>, a <<zero>> behaves th
 :: same as ''rotate3d(0, 1, 0, &lt;angle>)''.
 : <span class='prod'><dfn>rotateZ()</dfn> = rotateZ( [ <<angle>> | <<zero>> ] )</span>
 :: same as ''rotate3d(0, 0, 1, &lt;angle>)'', which is a 3d transform equivalent to the 2d transform ''rotate(&lt;angle>)''.
-: <span class='prod'><dfn>perspective()</dfn> = perspective( <<length [0,∞]>> )</span>
+: <span class='prod'><dfn>perspective()</dfn> = perspective( <<length [0,∞]>> | ''none'' )</span>
 :: specifies a <a href="#PerspectiveDefined">perspective projection matrix</a>. This matrix scales points in X and Y based on their Z value, scaling points with positive Z values away from the origin, and those with negative Z values towards the origin. Points on the z=0 plane are unchanged. The parameter represents the distance of the z=0 plane from the viewer. Lower values give a more flattened pyramid and therefore a more pronounced perspective effect. For example, a value of 1000px gives a moderate amount of foreshortening and a value of 200px gives an extreme amount.
 
 	If the depth value is less than ''1px'',
@@ -1378,9 +1378,12 @@ One translation unit on a matrix is equivalent to 1 pixel in the local coordinat
 			</div>
 
 	<li id="PerspectiveDefined">
-			A perspective projection matrix with the parameter <em>d</em> is equivalent to the matrix:
+			A perspective projection matrix with the parameter <var>d</var> is equivalent to the matrix:
 
-		$$\begin{bmatrix} 1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 & -1/d & 1 \end{bmatrix}$$
+			$$\begin{bmatrix} 1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 & -1/d & 1 \end{bmatrix}$$
+
+			If the parameter <var>d</var> is ''none'' it is treated as infinity
+			(and the resulting matrix is the identity matrix).
 
 </ul>