Added fixtures for tests requiring a GL context

- Added an empty test-module for Buffer.
- Also fixed missing header dependency in the X11 Display
  wrapper's header file.
Merge branch 'feature-test-fixtures' into develop
Update of the Spectra advanced example

- Added a new renderer.
- Added a new spectrum calculator.
- Also done some minor fixes in the existing examples.
Merge branch 'develop' of github.com:matus-chochlik/oglplus into develop
Merge branch 'develop' into feature-oalplus
Update of the Spectra advanced example