Reuses libheif's own LGPL/MIT-licensed with-alpha-512x512.heic sample
under both a .heic and .heif filename, verified to decode correctly
via heic-convert (the library the image format work will use). No
suitably-licensed HEVC/mif1-branded .heif-distinct sample exists that
is compatible with the pinned libheif-js version, and file-type detects
both extensions identically regardless of container brand, so the
duplicate content is sufficient to exercise the heif source-format
code path.
