Promote pixel offset multiplications to size_t in reformat.c

[uwezkhan]

This patch improves the safety of pixel offset calculations in src/reformat.c by ensuring that all byte offset multiplications are performed using size_t arithmetic.

In several places inside avifImageRGBToYUV and avifImageYUVAnyToRGBAnySlow, expressions such as:

i * rgbPixelBytes
uvI * yuvChannelBytes
i * 2

were evaluated using 32-bit integer arithmetic before being implicitly promoted to size_t for pointer arithmetic. Although current AV1 frame dimension limits prevent these expressions from overflowing in practice, relying on 32-bit intermediate arithmetic introduces unnecessary fragility.

This change explicitly casts loop indices to size_t before multiplication, for example:

(size_t)i * rgbPixelBytes
(size_t)uvI * yuvChannelBytes

This guarantees that offset computations are performed in the correct width type prior to pointer arithmetic. The update does not modify any control flow, logic, or behavior. It simply ensures arithmetic correctness and strengthens robustness in code that processes untrusted image data.

A total of 36 offset calculations were updated. No functional changes are expected.

[vrabaud]

This is a duplicate of #3072 ? Please close if so