Might fix #94729
Also tracked in https://bugs.chromium.org/p/skia/issues/detail?id=12717

This moves the call to initialize into the compare method of the Flutter[Pre/Post]SubmitFileComparators. Calling it in the constructor means we might be initializing when there are no image tests being executed.

It also adds a flag to keep track of whether or not an instance of the skia client has been initialized, so we can just skip it in that case too.

Also tidied up a bit while I was at it.

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