Now OpenGL can report its problems instead of shutting down the application.

Fix many doxygen warnings (due to missing info, old comments, typo ...) when building the doxygen doc.

Moved background color settings from GAL to RENDER_SETTINGS. Added RENDER_SETTINGS::TranslateColor() to convert between EDA_COLOR_T and COLOR4D.

Removed a few RecacheAllItems() calls, some of them changed to specific type recaching (using TYPE_COLLECTOR & VIEW_ITEM::ViewUpdate() ).
Removed OPENGL_GAL::SetStrokeColor().

template<> -> template <>
Some more reformatting according to uncrustify results.

Added flipping mode.

Smarter way of the overlay rendering (overlay is always refreshed, while cached&noncached targets only if the viewport or items have changed).

Fixed blending function for OpenGL compositing. Corrected documentation, removed unnecessary functions.

Code refactorization. VBO_CONTAINER is split to [NON]CACHED_MANAGER, GPU_MANAGER and VERTEX_MANAGER.

Removed most of deprecated OpenGL calls. Items used to be drawn in immediate mode now are drawn using vertex arrays.

Fixed drawing stroked semicircles using OpenGL backend.