Fix missing references to OffscreenCanvas

A few places that do expect a CanvasImageSource omit the OffscreenCanvas interface, while it is defined as being one in https://html.spec.whatwg.org/multipage/canvas.html#canvasimagesource
A couple of non-normative notes also were missing notes about OffscreenCanvas.

source

@@ -67192,6 +67192,8 @@ try {
 
    <li><code>HTMLCanvasElement</code> (<code>canvas</code> elements)</li>
 
+   <li><code>OffscreenCanvas</code></li>
+
    <li><code>ImageBitmap</code></li>
 
    <li><code>VideoFrame</code></li>
@@ -67281,6 +67283,9 @@ try {
   <p>When a <code>CanvasImageSource</code> object represents an <code>ImageBitmap</code>, the
   object's bitmap image data must be used as the source image.</p>
 
+  <p>When a <code>CanvasImageSource</code> object represents an <code>OffscreenCanvas</code>, the
+  object's bitmap must be used as the source image.</p>
+
   <p>When a <code>CanvasImageSource</code> object represents a <code>VideoFrame</code>, the object's
   pixel data must be used as the source image, and the source image's dimensions must be the
   object's <span>[[display width]]</span> and <span>[[display height]]</span>.</p>
@@ -67298,6 +67303,7 @@ try {
 
    <dt><code>HTMLCanvasElement</code>
    <dt><code>ImageBitmap</code>
+   <dt><code>OffscreenCanvas</code>
    <dd><p><var>image</var>'s bitmap's <span data-x="concept-canvas-origin-clean">origin-clean</span>
    flag is false.</p></dd>
   </dl>
@@ -70719,11 +70725,11 @@ interface <dfn interface>OffscreenCanvasRenderingContext2D</dfn> {
   access information (e.g. read pixels) from images from another origin (one that isn't the <span
   data-x="same origin">same</span>).</p>
 
-  <p>To mitigate this, bitmaps used with <code>canvas</code> elements and <code>ImageBitmap</code>
-  objects are defined to have a flag indicating whether they are <span
-  data-x="concept-canvas-origin-clean">origin-clean</span>. All bitmaps start with their <span
-  data-x="concept-canvas-origin-clean">origin-clean</span> set to true. The flag is set to false
-  when cross-origin images are used.</p>
+  <p>To mitigate this, bitmaps used with <code>canvas</code> elements, <code>OffscreenCanvas</code>
+  objects, and <code>ImageBitmap</code> objects are defined to have a flag indicating whether they
+  are <span data-x="concept-canvas-origin-clean">origin-clean</span>. All bitmaps start with their
+  <span data-x="concept-canvas-origin-clean">origin-clean</span> set to true. The flag is set to
+  false when cross-origin images are used.</p>
 
   <p>The <code data-x="dom-canvas-toDataURL">toDataURL()</code>, <code
   data-x="dom-canvas-toBlob">toBlob()</code>, and <code
@@ -70732,9 +70738,9 @@ interface <dfn interface>OffscreenCanvasRenderingContext2D</dfn> {
   cross-origin data.</p>
 
   <p>The value of the <span data-x="concept-canvas-origin-clean">origin-clean</span> flag is
-  propagated from a source <code>canvas</code> element's bitmap to a new <code>ImageBitmap</code>
-  object by <code data-x="dom-createImageBitmap">createImageBitmap()</code>. Conversely, a
-  destination <code>canvas</code> element's bitmap will have its <span
+  propagated from a source's bitmap to a new <code>ImageBitmap</code> object by <code
+  data-x="dom-createImageBitmap">createImageBitmap()</code>. Conversely, a destination <code
+  >canvas</code> element's bitmap will have its <span
   data-x="concept-canvas-origin-clean">origin-clean</span> flags set to false by <code
   data-x="dom-context-2d-drawImage">drawImage</code> if the source image is an
   <code>ImageBitmap</code> object whose bitmap has its <span