gauss_w_lut is a statically defined variable of type oclMat. The oclMat
destructor calls openCLFree() which via getInitializationMutex() accesses
the __module variable which has been statically defined in cl_context.cpp
Since the destruction order of statically defined variables in different
compilation units is undefined, it is possible that __module will
be destructed before gauss_w_lut, which would result in a segfault when
getInitializationMutex() is called while destructing gauss_w_lut.
In order to avoid this issue, we need to make gauss_w_lut a private
member of the HOGDescriptors class so we know it will be destroyed
before __module.
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