Reject and propagate invalid ISO gain-map metadata

[gregbenz]

Summary

• propagate HEIF/AVIF gain-map metadata parsing and conversion errors during probe
• reject reversed rational boost bounds that float conversion can conceal
• release libheif resources when direct AVIF/HEIF decoding encounters invalid metadata

Why

Current main validates converted gain-map descriptors, but HEIF/AVIF probing can still discard metadata errors and report success. Comparing the original rational values also catches invalid bounds that become indistinguishable after float rounding.

Testing

• confirmed both regressions fail on current main and pass with this change
• passed the complete 1,297-test suite
• passed focused AddressSanitizer tests

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