imgproc: fix bit-exact GaussianBlur() / sepFilter2D() (#15855)

* imgproc: fix bit-exact GaussianBlur() / sepFilter2D()

- avoid kernels with bad approximation
- GaussiabBlur - apply error-diffusion approximation for kernel (8-bit fraction)

* java(test): update features2d ref data

* test: update test_facedetect

modules/ts/include/opencv2/ts.hpp

@@ -428,6 +428,9 @@ protected:
     // updates progress bar
     virtual int update_progress( int progress, int test_case_idx, int count, double dt );
 
+    // dump test case input parameters
+    virtual void dump_test_case(int test_case_idx, std::ostream* out);
+
     // finds test parameter
     const CvFileNode* find_param( CvFileStorage* fs, const char* param_name );