Fix heap overflow in CopyDataAsFloatImpl by validating accessor bounds

[ztzat089-ctrl]

This PR fixes a heap buffer overflow (out-of-bounds read) in
CopyDataAsFloatImpl() in tiny_gltf_utils.h. The function did not
validate accessor.count, byte_stride, or buffer offsets before
reading data, which could allow reading beyond the allocated buffer
when processing a maliciously crafted glTF file.

This issue was reported to Google via the OSS VRP program
(Issue Tracker #543813782), and Google confirmed the vulnerability.
The fix adds bounds validation before the read.

Full test suite passes (425/425 tests).

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[ztzat089-ctrl]

Hello Google, please investigate and verify this vulnerability. Thank you very much.

[ztzat089-ctrl]

Hello Google, please investigate and verify this vulnerability. Thank you very much.

[igorvytyaz]

Thank you for making this contribution to Draco, @ztzat089-ctrl!