include/Graphics/GraphicsObject.h: Remove mColor, not used.
BackendGraphics: add method to draw polygons using a vertex vector.
sources/Backend/BackendGraphicsSDL.cpp: Use glrotated instead of glrotatef. compiler complains about casting double to float.
include/Graphics/BackendGraphics.h: Adding Virtual screen size
include/Graphics/BackendGraphics.h: change SetOrtho arguments and add SetOrtho2D method.
BackendGraphics: Replace 'SetOrtho' methods with 'Set2DProjection'
sources/Backend/BackendGraphicsSDL.cpp: in initGL() do not clear and swap buffers.

This is not needed. when a new frame needs to be drawn, clearBuffers() and Draw() methods will handle this.
sources/Backend/BackendGraphicsSDL.cpp
: in initGL() use one parameter to represent the color black.
sources/Backend/BackendGraphicsSDL.cpp: in init(). do not call initGL(). ResizeScreen() will do that.
Graphics/Texture: Minor fixes, remove direct reference to SDL_Surface as much as possible.