core: allow to construct CvMat from native cv::Mat pointer.

This patch solves the use-case when there is a mix of native code that
uses opencv and gocv.
In case that native code that utilizes opencv creates a CvMat and there
is a need to pass it to the golang environment without creating a copy.

The solution is to pass a native pointer to gocv and eventually call
cv::Mat copy constructor that performs shallow copy of the origin CvMat.

(See
https://docs.opencv.org/4.5.2/d3/d63/classcv_1_1Mat.html#a294eaf8a95d2f9c7be19ff594d06278e)
for more details.

core.cpp

@@ -77,6 +77,10 @@ Mat Mat_FromPtr(Mat m, int rows, int cols, int type, int prow, int pcol) {
     return new cv::Mat(rows, cols, type, m->ptr(prow, pcol));
 }
 
+Mat Mat_FromCVMat(Mat m) {
+    return new cv::Mat(*m);
+}
+
 // Mat_Close deletes an existing Mat
 void Mat_Close(Mat m) {
     delete m;

core.go

@@ -289,6 +289,14 @@ func NewMatFromBytes(rows int, cols int, mt MatType, data []byte) (Mat, error) {
 	return mat, nil
 }
 
+// NewMatFromCVMat returns a new Mat from native CVMat pointer. This can be used in case
+// some of the application code uses third party native libraries that return CVMat, so we can
+// construct gocv Mat from them. Data is not copied, Copy Constructor of cv::Mat is called.
+func NewMatFromCVMat(cvMatPtr unsafe.Pointer) Mat {
+	newMat := C.Mat_FromCVMat(C.Mat(cvMatPtr))
+	return Mat{p: newMat}
+}
+
 // Returns an identity matrix of the specified size and type.
 //
 // The method returns a Matlab-style identity matrix initializer, similarly to Mat::zeros. Similarly to Mat::ones.

core.h

@@ -247,6 +247,7 @@ Mat Mat_NewFromScalar(const Scalar ar, int type);
 Mat Mat_NewWithSizeFromScalar(const Scalar ar, int rows, int cols, int type);
 Mat Mat_NewFromBytes(int rows, int cols, int type, struct ByteArray buf);
 Mat Mat_FromPtr(Mat m, int rows, int cols, int type, int prows, int pcols);
+Mat Mat_FromCVMat(Mat m);
 void Mat_Close(Mat m);
 int Mat_Empty(Mat m);
 bool Mat_IsContinuous(Mat m);

core_test.go

@@ -9,6 +9,7 @@ import (
 	"runtime"
 	"strings"
 	"testing"
+	"unsafe"
 )
 
 func TestMat(t *testing.T) {
@@ -19,6 +20,22 @@ func TestMat(t *testing.T) {
 	}
 }
 
+func TestMatNativeCopy(t *testing.T) {
+	mat := NewMatWithSize(10, 10, MatTypeCV32F)
+	mat.SetTo(NewScalar(1, 2, 3, 4))
+	matData := mat.ToBytes()
+
+	mat2 := NewMatFromCVMat(unsafe.Pointer(mat.p))
+	mat.Close()
+	runtime.GC()
+
+	if !bytes.Equal(matData, mat2.ToBytes()) {
+		t.Error("TestMatNativeCopy: mats are not equal")
+	}
+
+	mat.Close()
+}
+
 func TestMatWithSizes(t *testing.T) {
 	t.Run("create mat with multidimensional array", func(t *testing.T) {
 		sizes := []int{100, 100, 100}