Fix caching of sample locations

The original caching logic for sample locations wishfully assumed that
the GPU would always use the same sample pattern for render targets
that had the same number of samples. It turns out we can't rely on
that. This change improves the caching logic to handle mismatched
simple patterns with the same count, and adds a unit test that
emulates different sample patterns observed on real hardware.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2111423002

Review-Url: https://codereview.chromium.org/2111423002

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