Merge pull request #25055 from dmatveev:dm/value_initialized_gmat

G-API: A quick value-initialization support GMat #25055

This PR enables `GMat` objects to be value-initialized in the same way as it was done for `GScalar`s (and, possibly, other types).

- Added some helper methods in backends to distinguish if a certain G-type value initialization is supported or not;
- Added tests, including negative.

Where it is needed:

- Further extension of the OVCV backend (#24379 - will be refreshed soon);
- Further experiments with DNN module;
- Further experiments with "G-API behind UMat" sort of aggregation.

In the current form, PR can be reviewed & merged (@TolyaTalamanov please have a look)

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [ ] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake

modules/gapi/include/opencv2/gapi/gmat.hpp

@@ -77,6 +77,18 @@ public:
      */
     GAPI_WRAP GMat();                       // Empty constructor
 
+    /**
+     * @brief Constructs a value-initialized GMat
+     *
+     * GMat may be associated with a buffer at graph construction time.
+     * It is useful when some operation has a Mat input which doesn't
+     * change during the program execution, and is set only once.
+     * In this case, there's no need to declare such GMat as graph input.
+     *
+     * @param m a cv::Mat buffer to associate with this GMat object.
+     */
+    GAPI_WRAP explicit GMat(cv::Mat m);     // Value-initialization constructor
+
     /// @private
     GMat(const GNode &n, std::size_t out);  // Operation result constructor
     /// @private

modules/gapi/include/opencv2/gapi/gscalar.hpp

@@ -54,12 +54,11 @@ public:
     /**
      * @brief Constructs a value-initialized GScalar
      *
-     * In contrast with GMat (which can be either an explicit graph input
-     * or a result of some operation), GScalars may have their values
-     * be associated at graph construction time. It is useful when
-     * some operation has a GScalar input which doesn't change during
-     * the program execution, and is set only once. In this case,
-     * there is no need to declare such GScalar as a graph input.
+     * GScalars may have their values be associated at graph
+     * construction time. It is useful when some operation has a
+     * GScalar input which doesn't change during the program
+     * execution, and is set only once. In this case, there is no need
+     * to declare such GScalar as a graph input.
      *
      * @note The value of GScalar may be overwritten by assigning some
      * other GScalar to the object using `operator=` -- on the