Refactor CalcFrameSsim to decompose overlapping 8x8 SSIM windows
(stepped by 4 pixels horizontally and vertically) into non-overlapping
4x4 pixel tiles across a 2-row rotating buffer (row0, row1), reducing
memory reads and pixel accumulation work by 4x:
- Ssim4x4Row_* computes 4 uint32_t statistics {sum_a, sum_b, sum_sq_ab,
sum_axb} per 4x4 pixel tile across a 4-row strip (also sized to fit
future 10-bit/12-bit formats like I010 without uint32_t overflow).
- SsimWindowRow_C combines top and bottom 4x4 tile strips across
adjacent horizontal pairs and returns the exact double SSIM sum,
producing bit-identical results to Ssim8x8_C.
- CalcFrameSsim rotates row0 and row1 (with thread-local buffers and
continuity tracking under _OPENMP) and falls back to Ssim8x8_C only if
buffer allocation fails.
- Implement Ssim4x4Row across all SIMD architectures:
- x86/x64: Ssim4x4Row_SSE2, Ssim4x4Row_AVX2, and Ssim4x4Row_AVX512BW
(with k-masked tail handling for 4..60 pixels) in both
compare_gcc.cc and compare_win.cc.
- ARM: Ssim4x4Row_NEON (compare_neon.cc, compare_neon64.cc),
Ssim4x4Row_NEON_DotProd (compare_neon64.cc), Ssim4x4Row_SVE2
(row_sve.h, row_sve.cc), and Ssim4x4Row_SME (row_sme.cc).
- RISC-V: Ssim4x4Row_RVV (row_rvv.cc).
- LoongArch: Ssim4x4Row_LSX (row_lsx.cc) and Ssim4x4Row_LASX
(row_lasx.cc).
- WebAssembly: Ssim4x4Row_WASMSIMD (compare_wasm.cc).
- Enable BenchmarkSsim_Opt and add BenchmarkSsim_Unaligned and
TestSsimAnyWidth in unit_test/compare_test.cc.
R=rrwinterton@gmail.com
Bug: None
Test: out/Release/libyuv_unittest --gtest_filter=*Ssim*
Change-Id: Ibb95ae74424ecdda073ab7ab10ab1cf610b121b6
