Add support for b4_SSE2 batched mode. #3647

Job	Run time
macos-13 C++17 py3.11 MacOS-13 llvm17	24m 15s
macos-14 C++17 py3.11 MacOS-14-ARM llvm17	8m 34s
Ubuntu Debug gcc7/C++17 llvm9 py3.7 oiio2.3 exr2.4 sse4 exr2.4	41m 19s
VFX2021 gcc9/C++17 llvm11 py3.7 exr2.5 oiio2.3 sse4 batch-b8avx2	16m 27s
Ubuntu gcc10/C++17 llvm10 oiio-release exr2.5 avx2	10m 3s
VFX2021 gcc9/C++17 llvm11 py3.7 exr2.5 oiio2.3 sse2 batch-b4sse2	20m 53s
Ubuntu latest releases gcc11/C++17 llvm16 exr3.2 py3.9 avx2 batch-b16avx512	22m 25s
VFX2022 gcc9/C++17 llvm13 py3.9 exr3.1 oiio-rel avx2	17m 55s
Ubuntu bleeding edge gcc12/C++17 llvm17 oiio/ocio/exr/pybind-master py3.10 avx2 batch-b16avx512	23m 12s
VFX2022 clang12/C++17 llvm12 oiio-master exr3.1 py3.9 avx2 batch-avx512	21m 57s
Ubuntu clang14/C++17 llvm14 exr3.1 py3.8 avx2 batch-b16avx512	25m 8s
VFX2022 icc/C++17 llvm14 py3.9 exr3.1 oiio-master avx2	28m 11s
VFX2022 icx/C++17 llvm14 py3.9 exr3.1 oiio2.3 avx2	21m 20s
Ubuntu clang-format	4m 16s
VFX2023 gcc11/C++17 llvm15 py3.10 exr3.1 oiio-rel avx2	19m 57s
VFX2023 GPU Cuda11 gcc11/C++17 llvm15 py3.10 exr3.1 OIIO-master avx2	6m 18s
VFX2021 oldest everything gcc9/C++17 llvm9 py3.7 oiio2.3 no-simd exr2.4	9m 12s
VFX2022 sanitizers	28m 41s
VFX2023 abi check	13m 35s
	6h 3m 38s

Provide feedback