[Editorial] Fix missing references to OffscreenCanvas as CanvasImageSource

[Kaiido]

A few places that do expect a CanvasImageSource omit the OffscreenCanvas interface case, 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.

Since the WebIDL is already enforcing that OffscreenCanvas is part of CanvasImageSource, this is merely an editorial change.

• At least two implementers are interested (and none opposed):
  • N/A, this is a non-normative change.
  • …
• Tests are written and can be reviewed and commented upon at:
  • N/A
• Implementation bugs are filed:
  • N/A
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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/canvas.html ( diff )

[domenic]

Some of these missing definitions feel a bit more substantial, so I'll call this a non-editorial bugfix.