Merge pull request #28217 from satyam102006:feature/dis-coarsest-scale

[video] Add setCoarsestScale to DISOpticalFlow (Fixes #25068) #28217

### Description
This PR addresses issue #25068 regarding the number of scales in `DISOpticalFlow`.

Currently, `DISOpticalFlow` automatically computes the `coarsest_scale` based on the image size. While generally effective, this behavior can cause errors in specific use cases (e.g., mechanics/speckle pattern analysis) where high pyramid levels degrade quality.

This change introduces a manual override:
- Added `setCoarsestScale(int val)` and `getCoarsestScale()` to the public API.
- Updated `DISOpticalFlowImpl::calc` and `ocl_calc` to use the user-defined scale if set.
- Preserved the existing "automatic" behavior as the default (when set to -1).

### Changes
- **`modules/video/include/opencv2/video/tracking.hpp`**: Added virtual method declarations.
- **`modules/video/src/dis_flow.cpp`**: Implemented `set/getCoarsestScale` in `DISOpticalFlowImpl` and updated calculation logic.
- **`modules/video/test/test_OF_accuracy.cpp`**: Added `DenseOpticalFlow_DIS.ManualCoarsestScale` regression test.


Fixes #25068

modules/video/include/opencv2/video/tracking.hpp

@@ -678,6 +678,16 @@ public:
     /** @copybrief getFinestScale @see getFinestScale */
     CV_WRAP virtual void setFinestScale(int val) = 0;
 
+    /** @brief Sets the coarsest scale
+    @param val Coarsest level of the Gaussian pyramid on which the flow is computed.
+    If set to -1, the auto-computed coarsest scale will be used.
+    */
+    CV_WRAP virtual void setCoarsestScale(int val) = 0;
+
+    /** @brief Gets the coarsest scale
+    */
+    CV_WRAP virtual int getCoarsestScale() const = 0;
+
     /** @brief Size of an image patch for matching (in pixels). Normally, default 8x8 patches work well
         enough in most cases.
         @see setPatchSize */