[css-transforms-2] perspective() clamping also applies to resolved value and interpolation

Define that clamping of values of the perspective() function smaller
than 1px also applies to the resolved value and to interpolation, as
resolved in w3c#6320.

Fixes w3c#6320, fixes w3c#6346.

Author: dbaron

css-transforms-2/Overview.bs

@@ -987,7 +987,14 @@ In the following <dfn export>3d transform functions</dfn>, a <<zero>> behaves th
 :: 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'',
-	it must be treated as ''1px'' for the purpose of rendering.
+	it must be treated as ''1px'' for the purpose of rendering,
+	for computing the <a>resolved value</a> of 'transform',
+	and when used as the endpoint of
+	<a href="#interpolation-of-transform-functions">interpolation</a>.
+
+	Note: The intent of the above rules on values less than ''1px'' is
+	that they cover the cases where the ''perspective()'' function
+	needs to be converted into a matrix.
 
 
 Transform function primitives and derivatives {#transform-primitives}