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

Screen size is saved in VECTOR2I instead of VECTOR2D.

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

Fixes missing worksheet layout when there is no PCB loaded and all random brightness changes for the Cairo rendering backend.

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).

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

Fixed drawing stroked semicircles using OpenGL backend.

Added VBO_CONTAINER as a faster storage for vertices (OPENGL_GAL), tuned for exchanging data with GPU.

Removed a few unnecessary variables and fields from OPENGL_GAL.
Added function GAL::ClearCache() for freeing memory used by cached items.
Fixed a few memory leaks (tesselator, PAINTER's settings & VIEW_ITEM's groups).
Changed a few functions into inlines.