Moved the rendering code out to a separate directory in the hope that it can someday be completely decoupled from the rest of the library and be expanded to an awesome 2D on 3D library.

--HG--
rename : src/video/windows/SDL_d3drender.c => src/render/direct3d/SDL_d3drender.c
rename : src/video/SDL_renderer_gl.c => src/render/opengl/SDL_renderer_gl.c
rename : src/video/SDL_renderer_gles.c => src/render/opengles/SDL_renderer_gles.c
rename : src/video/SDL_renderer_sw.c => src/render/software/SDL_renderer_sw.c