Workaround for misalignment of SubmapSlice and Eigen data structures in heterogeneous builds
#1910
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Due to different structure alignment rules in different compilers (or even the same compiler in different compiling contexts), the
SubmapSlicestructure defined in ./cartographer/io/submap_painter.h comes intocartographer_rosor another external project misaligned by 48 bytes because of differentsizeof()return values (224 vs 256 bytes). This behaviour is proven when testing Cartographer compiled using Clang 15 and GCC 11.3 on Mint Linux 22 with a customized build of ROS. The reason is deeply sitting problem with implementation-defined STL shipped with different kernels and build pipelines. This pull request adds a semi-portable workaround for this issue forcing alignment ofSubmapSlicestructure to 32 bytes as is required by GCC.#pragma packfree()oncairo_data#1909