PNG: Avoid allocations when disposing APNG frames

[pnn64]

This PR changes APNG background and previous-frame disposal to operate directly on the underlying image rows. Previously, these paths created temporary buffers before clearing or restoring a region, resulting in unnecessary allocations and memory copies. The new implementation performs the same work directly in the frame buffers without changing the public API or using unsafe code.

The existing six WPT APNG reference tests pass. Local measurements produced the following results:

Disposal mode	Allocations	Allocated bytes	Reallocations	Latency
Background	20 → 19	331,504 → 306,928	10 → 1	~30% lower
Previous	20 → 19	315,120 → 306,928	1 → 1	~16% lower

The benchmarks were run locally and are not included in this PR.

Tested with cargo test --test reference_images -- wpt and cargo clippy --lib -- -D warnings.

AI disclosure: I used Codex with GPT-5.6 Sol at xhigh reasoning effort to help identify the allocation, implement the optimization, and construct the local benchmarks. I reviewed and tested the resulting change.