G-API: replace GAPI_Assert() with 'false' and '0' to GAPI_Error()
* gapi: GAPI_Error() macro
* gapi: replace GAPI_Assert() with 'false' and '0' to GAPI_Error()
* build: eliminate 'unreachable code' after CV_Error() (MSVC 2015)
* build: eliminate 'unreachable code' warning for MSVS 2015/2017
- observed in constructors stubs with throwing exception
1) Document GFrame/MediaFrame (and also other G-API types)
- Added doxygen comments for GMat, GScalar, GArray<T>, GOpaque classes;
- Documented GFrame and its host-side counterpart MediaFrame;
- Added some more notes to the data type classes.
2) Give @brief descriptions to most of the cv::gapi::* namespaces
3) Make some symbols private
- These structures are mainly internal and shouldn't be used directly
G-API: Extend MediaFrame to be able to extract additional info besides access
* Extend MediaFrame to be able to extract additional info besides access
* Add default implementation for blobParams()
* Add comment on the default blobParams()
