fix typos in CanvasTextDrawingStyles and remove getter dependencies

[Kaiido]

This is a follow-up on #7378.
It fixes a couple of typos, and adds the current filter and the current composition operator in the list of saved values from CanvasState, as per #7378 (comment), since these already have their own internal value being defined.
Other attributes would require a lot more work to define (and use) their internal values.

• At least two implementers are interested (and none opposed):
  • This should be an editorial change only (Update letterSpacing and wordSpacing to accept String #7378 already did update fillStyle and strokeStyle)
• Tests are written and can be reviewed and commented upon at:
  • N/A
• Implementation bugs are filed:
  • N/A
    (See WHATWG Working Mode: Changes for more details.)

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/canvas.html ( diff )

[domenic]

Thank you, this is great! The "global composition operator" is a bit of a strange case (the getter/setter steps are not defined, but somehow the backing concept is defined and used?). But I think this is pretty clear.

[Kaiido]

Yes gCO is a bit weird indeed.
I guess it should now link directly to https://drafts.fxtf.org/compositing/#canvascompositingandblending and it should use some kind of "parse as a CSS <blend-mode> or <composite-mode>" algorithm instead of the current wording, but I'm not sure how the case-sensitivity would play there...
Do you want me to try to write some fix around that when I get some time, or just start a new issue about that, or is it "fine-enough" like that?

[domenic]

A fix would be lovely. My first instinct is that we don't need a particular issue tracking it (as opposed to just a general sense that lots of parts of the canvas spec aren't following modern best-practices), but it might be helpful for discussion or for your tracking, so feel free.

It does seem like we probably can't take the parse/serialize route there, at least not without changes.

Another thing we could do is link to the external definitions for source-over and copy instead of redefining them.