Use new SkImages namespace instead of legacy SkImage static functions

[kjlubick]

In https://skia-review.googlesource.com/c/skia/+/661059, Skia added a new SkImages namespace and deprecated using static functions on SkImage as part of an effort to separate the SkImage class and the GPU backends.

This PR replaces all old calls with the new calls, which should be functionally the same. It makes sure SkImage.h and SkImageGanesh.h are #included where appropriate.

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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