canvas: Reimplement masking

[Herschel]

Reimplement masking on the canvas backend using the native CanvasRenderinContext2d.clip method.

When I first implemented masking in canvas, I didn't think clip could handle all of the nuances of Flash masking, so I ended up with a hacky+slow solution of drawing to off-screen canvases and using globalCompositeOperation to blend onto the main canvas. But I think we can handle nearly all of the cases natively using clip now:

• clip(Path2D) is now supported in all major browsers.
• Strokes aren't considered when masking. Originally I was worried that strokes could mask content as well, which would be impossible with clip. But Flash operates the same as canvas and omits strokes.
• "Layered" art (multiple layers in a single Shape) is still a problem.
  • The layers should be unioned to generate the mask area, but with the canvas API, we can only really intersect using successive clip calls.
  • I settled on using a "non-zero" winding rule to handle this case, but this won't be completely correct in other cases (such as self-intersecting shapes).
  • But Flash IDE actively avoids exporting mask art in the above ways (multiple layers or self-intersecting), because the Flash Player itself doesn't handle these cases completely correctly either, sometimes producing artifacts. It's still possible to hit these cases using the drawing API + dynamic masks though. This should be rare, and hopefully the non-zero winding rule operates decently enough in these cases.

Fixes Gridlock drawing a blank screen in canvas backend (#6976).

[Toad06]

This looks good to me, I'm also impressed about the performance improvement in almost all files. For instance, Ultimate Flash Sonic was running pretty smoothly when played in windowed mode on Newgrounds but was really too slow and so unplayable with larger dimensions or in fullscreen mode. Now it's always smooth. :)

While I'm at it, I'd like to mention that the issue I mentioned here showed up again. This is not related to this PR but one of the following commits: nightly-2022-05-18...nightly-2022-05-20 (likely #7029?). Firefox and Chrome on Windows are both affected (canvas render only). The issue is particularly noticeable in Super Mario 63, in the first / tutorial level.

[Herschel]

Thanks! Will fix the bitmap issue in another PR.