A draw whose key has no entry captured its stages, built a variant per
stage (a copy of every captured region) and inserted a draw entry. A key
that never comes back paid for the variants and the entry for nothing,
and its entry pushed a live one out of the cache. Draws whose user data
changes every frame are such keys.
The driver now remembers the keys it has seen once, in a table of
four-way buckets holding four times the cache's size in keys. A key gets
its entry at its second sight; a key that already has an entry still
takes new variants at once. A full bucket replaces its oldest key, so
keys that share a bucket do not push each other out until more than four
of them come between two sights of one.
Test: agc_driver_draw_cache_reuse draws three times. The first two draws
find no entry, one entry is inserted, and the third draw hits it. A
sighting table of one bucket keeps two keys drawn in turn and replaces
its oldest key when full.
0adbdcf45c