SKAutoCanvasRestore avoid crash at Dispose when canvas was already disposed

[taublast]

Description of Change

Have catched this few times myself: might be my bad coding style, but nevertheless SKAutoCanvasRestore should not crash app if the canvas inside was already disposed for any reason whatever. So checking to avoid that inside SKAutoCanvasRestore Dispose method.

Bugs Fixed

SKAutoCanvasRestore could crash if GC-ed/disposed after the containing canvas native Handler was already released while canvas field still not NULL and Restore() would be called, then crash app accessing Handle 0.

EDIT: also added safety checks to mappers (second commit), after was running n Debug and catched landing into mapper code with a NULL handler parameter when navigating between pages with accelerated canvases. Would guess there want be any harm for adding those too to solve occasional very rare and random crashes.

• Fixes

No known issues identified, still could be related to some unknown random crashes.

No Api changes, a small code change, please see 1 file change below.

Required skia PR

None.

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Merged related skia PRs
• Changes adhere to coding standard
• Updated documentation

[taublast]

omg. might be the fix for the navigating away from page with skglview crash.

EDIT:

Yeah! It doesn't crash when i just comment this out! So when we merge i could uncomment :)

  //using (new SKAutoCanvasRestore(_retainedSurface.Canvas, true))
  {
      OnPaintSurface(new(_retainedSurface, _renderTarget, SurfaceOrigin, ColorType));
  }

Re-EDIT:

Ha just realized i can just use a custom-fixed SKAutoCanvasRestore meanwhile..

[mattleibow]

Thanks for the continued work!

[mattleibow]

/azp run

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[mattleibow]

/azp run

[azure-pipelines]

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