[css-sizing-3] Simplify the sizing diagrams and put them into the spec.

Author: tabatkins

css-sizing-3/Overview.bs

@@ -88,6 +88,12 @@ Terminology</h2>
 			a <a>block size</a> and/or <a>inline size</a>;
 			alternatively a width and/or height.
 
+			<figure>
+				<img style="max-width: 49%;" src="images/sizing-ltr-tb.svg"><img style="max-width: 49%;" src="images/sizing-ttb-rl.svg">
+				<figcaption>Whether the <a>width</a> or <a>height</a> corresponds
+				to an <a>inline size</a> or <a>block size</a> depends on the <a>writing mode</a>.</figcaption>
+			</figure>
+
 		<dt><dfn>inner size</dfn>
 		<dd>
 			The <a href="https://www.w3.org/TR/css2/box.html#box-dimensions">content-box</a> size of a <a>box</a>.