Started release-0.55.0
Multiple changes in Context and ClientContext

- Added buffer clearing operations.
- Added blend color stack.
- Refactoring of Context.
- Minor updates in an example.
Updated the changelog and release notes
Updated the news feed source
Merge branch 'release-0.55.0' into develop
Merge branch 'develop' into develop-with-eagine

Conflicts:
	include/oglplus/client/viewport.hpp
	include/oglplus/context/buffer_clearing.hpp
	include/oglplus/context/buffer_masking.hpp