Two defects in the HEIF/AVIF sniffing added by this PR.
1. Major-brand-only detection mislabeled AVIF as HEIC.
AVIF encoders routinely stamp the generic still-image brand 'mif1' as the
MAJOR brand and declare the AV1 codec only in the compatible-brand list, so
every mif1-major file was reported as HEVC-coded HEIC. Parse the
compatible-brand list (bounded by the declared ftyp box size, so an 'avif'
token in a following box can't upgrade a real HEIC) and let AV1 brands win
when both families appear.
2. AVIF was rejected outright when pillow-heif was missing.
Pillow >= 11.3 bundles a native AvifImagePlugin, while pillow-heif wheels
are commonly built with no AV1 codec at all (libheif_info() reports
AVIF: ''). Gating AVIF on a pillow-heif import therefore refused files
Pillow could already decode, and pointed the user at a library that cannot
decode them. Registration is now best-effort: the decode attempt is the
arbiter, and failures emit codec-specific guidance naming the backend that
actually serves that format.
Tests: mif1+avif and mif1+av01 regressions, a box-size bound guard, a
truncated/bogus-size guard, AVIF-converts-without-pillow-heif, and AVIF error
text. Verified against real pillow-heif-encoded HEIC and Pillow-encoded AVIF
files, not just synthetic headers; both new brand tests fail if the
compatible-brand scan is reverted.
Sam Painter
f5a7dc14c7