feat: support Windows MSVC builds

[xnorpx]

Summary

• update vendored Netflix/VMAF master to f85a853692a8c730d0270cd733c8bb30b5b93b7c
• add native Windows MSVC support with private Windows threading and runtime-dispatched AVX2/AVX-512 kernels
• propagate host Rust CPU settings into libvmaf C/C++ compilation; -C target-cpu=native now selects host-native compiler tuning while portable builds retain runtime dispatch
• recreate Meson output on build-script reruns so CPU flags cannot become stale
• run real two-thread JPEG scoring on every supported host
• apply local patches automatically from vendor_vmaf.py

Individual upstream patches

• ya_getopt.patch - Add ya_getopt, a 2-clause-BSD-licensed implementation of getopt_long() Netflix/vmaf#1410
• msvc_no_vla.patch - MSVC: Avoid the use of variable-length arrays Netflix/vmaf#1428
• avx2_simd_portability.patch - AVX2: Improve SIMD portability by replacing non‑portable indexing and casts Netflix/vmaf#1475
• vif_void_pointer.patch - Fix UB in void* pointer arithmetic and memory leak in VIF init Netflix/vmaf#1476

Local patches keep the private pthread layer, MSVC SIMD build wiring, FMA-safe AVX2 dispatch, and checked VIF copy-size arithmetic separate.

Validation

• regenerated current VMAF through the complete ordered patch series
• python3 check_vmaf_version.py
• python3 -m py_compile vendor_vmaf.py check_vmaf_version.py
• actionlint .github/workflows/*.yml
• cargo fmt --all -- --check
• five direct build-script configuration tests
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --no-default-features
• cargo test --all-features (two native VMAF worker threads)
• DOCS_RS=1 RUSTDOCFLAGS="-D warnings" cargo doc --all-features --no-deps
• cargo package --allow-dirty
• CFLAGS=-Werror=vla cargo check --no-default-features
• RUSTFLAGS="-C target-cpu=native" cargo check --all-features; verified the C command contains Meson release -O3 and -mcpu=native
• cargo ndk --platform 24 --target armeabi-v7a --link-builtins --link-libcxx-shared test --lib --no-run (scalar ARMv7 feature kernels)