Adds impeller gl context clear guard - #187332

issue: #166248

Speculative fix for #166248.

This ports over one of the safety guards from Skia over to Impeller:

This makes sure that if the context is not the current context when attempting to clear, it is a noop.

Pre-launch Checklist

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• I added new tests to check the change I am making, or this PR is test-exempt.
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• All existing and new tests are passing.

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