Adding getStdAllocator() to cv::cuda::GpuMat

To be on par with `cv::Mat`, let's add `cv::cuda::GpuMat::getStdAllocator()`
This is useful anyway, because when a user wants to use custom allocators, he might want to resort to the standard default allocator behaviour, not some other allocator that could have been set by `setDefaultAllocator()`

modules/core/include/opencv2/core/cuda.hpp

@@ -118,6 +118,7 @@ public:
     //! default allocator
     CV_WRAP static GpuMat::Allocator* defaultAllocator();
     CV_WRAP static void setDefaultAllocator(GpuMat::Allocator* allocator);
+    CV_WRAP static GpuMat::Allocator* getStdAllocator();
 
     //! default constructor
     CV_WRAP explicit GpuMat(GpuMat::Allocator* allocator = GpuMat::defaultAllocator());