The cache estimated an entry at channels * 4 bytes a pixel, four times
too much for 8-bit, always kept a single oversize entry, recovered a
poisoned lock by disabling itself, and nothing ever cleared it.
Count each entry's real tile bytes once when it is cached, and do not
cache a source larger than the whole budget (as the stage cache does).
Web builds keep 4 entries and 384 MiB instead of 32 and 1 GiB, and the
smart filter stage cache 256 MiB instead of 768 MiB. A poisoned lock
is recovered rather than ignored.
Edit > Purge > All now clears the cache and reports it, and enables on
it alone. Closing a document that has smart objects clears it too:
entries are shared by content hash, so other documents just decode
again on their next re-render.
