fmt.Sprintf ran on every newly-decoded image even with Debug off, since
Go evaluates log arguments regardless of level. Separate width/height
fields also query better than a "300x300" string.
Correction to e0f1acd1a's message: it said "all 40" messages carry the
prefix. The package has 68 log call sites and 62 distinct messages; 40
was only what the test suite happened to exercise. The sweep itself was
complete -- zero unprefixed messages remain.
e646ce5065