A stale storage snapshot was compared whole with the guest bytes and, on
any difference, copied whole into a new buffer, although few of the
tracker blocks stamped since the copy usually hold different bytes.
Only the 64 KiB tracker blocks stamped since the snapshot's generation
are compared now. When some differ and the cache holds the only
reference (no recorded draw binds the buffer), those blocks are copied
into it in place; otherwise the range is copied again as before.
agc_driver_recorder_tests drawSnapshotPatchTests: changed bytes in
several blocks and a whole changed block patch the snapshot nothing else
holds, an unchanged one is reused, and one a recorded draw still holds is
copied again with its bytes untouched.
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