Merge pull request #25382 from savuor:rv/fix_mesh_load

Fix mesh loading for texture coordinates and face indices #25382

### This PR changes

* Texture coordinates were stored incorrectly (3-channel array is read as if there were 2 channels), fixed
* Faces were pushed back to the output array instead of indexed writing which produced a lot of empty faces, fixed
* A set of ground truth tests were added to cover these issues
* `std::vector<cv::Mat>` support added for `saveMesh()` which is required for Python bindings
* More command line args were added to rasterization test data generator

### 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
- [x] 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/3d/test/test_rendering.cpp

@@ -516,13 +516,11 @@ protected:
 
         if (shadingType != RASTERIZE_SHADING_WHITE)
         {
+            // let vertices be in BGR format to avoid later color conversions
+            // mixChannels() does not support in-place operation
             colors = Mat(modelData.colors);
             colors.convertTo(colors, ftype);
-            // let vertices be in BGR format to avoid later color conversions
-            // cvtColor does not work with 1d Mats
-            std::vector<Mat> xyz;
-            cv::split(colors, xyz);
-            cv::merge(std::vector<Mat>{xyz[2], xyz[1], xyz[0]}, colors);
+            cv::mixChannels(colors.clone(), colors, {0, 2, 1, 1, 2, 0});
         }
 
         indices = Mat(modelData.indices);