ovis: add software-renderer based unit testing

[paroj]

the last Ogre release added a self-contained Software Rendering backend, which eases testing:
https://www.ogre3d.org/2021/02/12/ogre-1-12-11-released#Software_RenderSystem

this adds unit-tests for the last two bugs I fixed in ovis. If Ogre will be installed on the runners, they could also be executed as part of the opencv test-suite.

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
• n/a There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

[alalek]

Thank you!