The room estimate for a large PNG read a row at a time counted 32 bytes
a column. The png reader also keeps at least 128 KiB of rows already
read, the 32 KiB deflate window and a growth step in its stream buffer,
and up to four rows for wide images, so a decode could be approved with
less room than reading takes. The estimate now comes from the PNG
header's row length and these buffers, and a PNG is read a row at a
time only while that estimate fits the decode allowance.