[css-overflow-3][css-background-3] Define rounding of clipping region established by 'overflow'.

This formally defines the rounding of the clipping region established by
overflow, which was previously defined (other than for 'overflow:clip')
only in a "For example" in the backgrounds spec.

It writes that definition in order to implement the resolution in #7434
(regarding mixes of 'visible' and 'clip' on different axes), and also to
correctly account for overflow-clip-margin.

Finally, it adjusts the aforementioned example for the resolution
in #7434 as well.

Fixes #7434.

Author: dbaron

css-backgrounds-3/Overview.bs

@@ -1657,7 +1657,8 @@ to the border, padding, or content edge
 must clip to their respective curves.
 For example,
 backgrounds clip to the curve specified by 'background-clip',
-'overflow' values other than ''visible'' to the curved padding edge,
+'overflow' values other than ''visible'' to the curved padding edge
+(when 'overflow' on both axes is not ''visible''),
 [=replaced element=] content to the curved content edge,
 pointer events to the curved border edge,
 etc.

css-overflow-3/Overview.bs

@@ -537,6 +537,21 @@ Managing Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</h3
 	compute to ''overflow/auto''/''hidden'' (respectively)
 	if one of 'overflow-x' or 'overflow-y' is neither ''visible'' nor ''overflow/clip''.
 
+	As mentioned in [[css-backgrounds-3#corner-clipping]],
+	the clipping region established by 'overflow' can be rounded:
+
+	* When 'overflow-x' and 'overflow-y' compute to
+		''overflow/hidden'', ''overflow/scroll'', or ''overflow/auto'', 
+		the clipping region is rounded
+		based on the border radius,
+		adjusted to the [=padding edge=],
+		as described in [[css-backgrounds-3#corner-shaping]].
+	* When both 'overflow-x' and 'overflow-y' compute to ''overflow/clip'',
+		the clipping region is rounded as described in [[#overflow-clip-margin]].
+	* However, when one of 'overflow-x' or 'overflow-y' computes to ''overflow/clip''
+		and the other computes to ''overflow/visible'',
+		the clipping region is not rounded.
+
 	If the computed value of 'overflow' on a <a>block box</a>
 	is neither ''overflow/visible'' nor ''overflow/clip'' nor a combination thereof,
 	it [=establishes an independent formatting context=] for its contents.