Changed the used framebuffer object extension from ARB_framebuffer_object to EXT_framebuffer_object (compatibility reasons).

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

Added error messages for OpenGL framebuffer errors. This should eliminate quiet segfaults in case of drivers that cannot handle multiple attachment points for framebuffers.

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).

Now it is more verbose and the status is checked at the right moment (previously it was fine with Linux, but on Windows it showed errors).

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.