Reject iloc with total item size exceeding 16x file size

[littlepig12345]

avifDecoderParse() trusts the sum of item sizes declared in iloc. Multiple items can point at the same extent, so N items pointing at the same L-byte region cause N * L bytes to be allocated. A small file can therefore trigger a disproportionate allocation.

Test results:

Before the patch:

• Peak WorkingSet: ~8015 MB
• Elapsed: ~4.3 s
• Result: "Out of memory"

After the patch:

• Peak WorkingSet: ~19.7 MB
• Elapsed: ~0.2 s
• Result: "Not implemented" (total item size exceeds 16x file size; overlapping extents are not supported)

The check compares the total declared item size against the file size, regardless of how the amplification is constructed.

Overlapping extents are allowed by ISO/IEC 14496-12 but are not supported by libavif for allocation purposes, so the check returns AVIF_RESULT_NOT_IMPLEMENTED.

This follows up on the overlapping extents case mentioned by @y-guyon in #3375.