[canvaskit] Make the context current before disposing it

[harryterkelsen]

When we delete a Surface context (for example because it has resized) we must make sure that it is the current context or Skia might throw an exception while deleting resources associated with that context.

Fixes flutter/flutter#91881

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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[ditman]

Do we still want to land this?

[harryterkelsen]

Do we still want to land this?

Yes. For some reason I thought I already landed it

[ditman]

Thanks @hterkelsen!!