fix(ImageBuf): IB::pixeltype() did not always return the right value #7130

Job	Run time
(old) VP2021 gcc9/C++17 py3.7 exr3.1 ocio2.0	13m 56s
(old) VP2021 clang10/C++17 avx2 exr3.1 ocio2.0	14m 21s
(old) VP2022 gcc9/C++17 py39 exr3.1 ocio2.1	12m 54s
(old) VP2022 clang13/C++17 py39 avx2 exr3.1 ocio2.1	13m 19s
(old) oldest gcc9.3/C++17 py3.7 exr-3.1	14m 13s
(old) hobbled gcc9.3/C++17 py3.7 exr-3.1 no-sse	14m 0s
Windows-2019 VS2019 / c++17 py3.9	49m 23s
Windows-2022 VS2022 / c++17 py3.9	39m 12s
MacOS-13-Intel aclang14/C++17/py3.11 / clang++ c++17 py3.11	19m 48s
MacOS-14-ARM aclang15/C++20/py3.12 / clang++ c++20 py3.12	9m 49s
MacOS-15-ARM aclang16/C++20/py3.13 / clang++ c++20 py3.13	9m 29s
ABI check / c++17 py3.11	12m 54s
bleeding edge gcc14 C++20 py3.12 OCIO/libtiff/exr-main avx2 / g++-14 c++20 py3.12	16m 39s
all local builds gcc12 C++17 avx2 exr3.2 ocio2.3 / g++-12 c++17 py3.10	18m 22s
clang14 C++20 avx2 exr3.1 ocio2.1 / clang++ c++20 py3.8	15m 47s
clang-format / c++17 py3.10	7m 31s
debug gcc9/C++17, sse4.2, exr3.1 / g++-9 c++17 py3.8	18m 13s
latest releases gcc13 C++20 py3.12 avx2 exr3.3 ocio2.4 / g++-13 c++20 py3.12	17m 6s
static libs gcc9 C++17 exr3.1 / g++-9 c++17 py3.8	14m 54s
VFX2023 icc/C++17 py3.10 exr3.1 ocio2.1 qt5.15 / c++17 py3.10	20m 11s
VFX2023 gcc11/C++17 py3.10 exr3.1 ocio2.2 / c++17 py3.10	11m 40s
VFX2023 icx/C++17 py3.10 exr3.1 ocio2.2 qt5.15 / icpx c++17 py3.10	14m 22s
VFX2024 clang/C++17 py3.11 exr3.2 ocio2.3 / clang++ c++17 py3.11	12m 8s
VFX2024 sanitizers / clang++ c++17 py3.11	41m 1s
VFX2024 gcc11/C++17 py3.11 exr3.2 ocio2.3 / c++17 py3.11	12m 46s
	7h 23m 58s

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