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

VIEW_GROUP now does not change depth during drawing. Introduced functions for manipulating stored items.
FIxed problem of overlay being covered by other layers while panning.
Few minor fixes.

Used in highlighting/routing code.

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.