Added because there was no method to get Mat data directly in Objective-C or Swift.

Usage example in Swift:

let mat = Mat.zeros(Size(width: 256, height: 256), type: CvType.CV_8UC4)
let dataPointer = mat.dataPointer() // <- UnsafeMutablePointer<UInt8>
let data = Data(bytes: dataPointer, count: bytesPerPixel * pixels)

Pull Request Readiness Checklist

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

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work

force_builders=Custom Mac
build_image:Custom Mac=osx_framework

buildworker:iOS=macosx-1