Fix ovis module building error with OGRE 1.12.9

[twistedfall]

Hi, this fixes the build error that I started to get while building the ovis module with OGRE 1.12.9 since OpenCV 4.6.0.
The error was:

/home/pro/projects/opencv-lib/opencv-4.6/opencv_contrib-4.6.0/modules/ovis/src/ovis.cpp: In member function ‘virtual void cv::ovis::WindowSceneImpl::setBackground(cv::InputArray)’:
/home/pro/projects/opencv-lib/opencv-4.6/opencv_contrib-4.6.0/modules/ovis/src/ovis.cpp:456:65: error: cannot convert ‘std::__shared_ptr_access<Ogre::Material, __gnu_cxx::_S_atomic, false, false>::element_type’ {aka ‘Ogre::Material’} to ‘const String&’
  456 |                 RTShader::ShaderGenerator::DEFAULT_SCHEME_NAME, *bgplane->getMaterial());
      |                                                                 ^~~~~~~~~~~~~~~~~~~~~~~
      |                                                                 |
      |                                                                 std::__shared_ptr_access<Ogre::Material, __gnu_cxx::_S_atomic, false, false>::element_type {aka Ogre::Material}
In file included from /usr/include/OGRE/RTShaderSystem/OgreRTShaderSystem.h:32,
                 from /usr/include/OGRE/Bites/OgreSGTechniqueResolverListener.h:32,
                 from /usr/include/OGRE/Bites/OgreApplicationContextBase.h:41,
                 from /usr/include/OGRE/Bites/OgreApplicationContext.h:33,
                 from /home/pro/projects/opencv-lib/opencv-4.6/opencv_contrib-4.6.0/modules/ovis/src/ovis.cpp:7:

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[alalek]

/cc @paroj

[paroj]

you should also pass RESOURCEGROUP_NAME, so the Material is uniquely identified

[twistedfall]

Done