decoder: avoid signed overflow in texture prediction

[Alb3e3]

Summary

The minimized OSS-Fuzz testcase for issue 553263223 reaches the portable texture-coordinate predictor with untrusted values that overflow signed int64_t during scaled UV addition. UBSan reports the overflow in VectorD::operator+.

Perform the intermediate vector arithmetic in uint64_t before converting back to the signed vector. Valid bitstreams retain their results, while malformed geometry no longer invokes signed-overflow undefined behavior during decoding.

Validation: rebuilt Draco with AddressSanitizer and UndefinedBehaviorSanitizer and replayed the minimized point-cloud and mesh decoder harnesses without findings.

This is a Google Patch Rewards OSS-Fuzz Tier 2 patch. AI assistance was used and is disclosed.