Converted David Ludwig's D3D11 renderer to C and optimized it.

The D3D11 renderer is now slightly faster than D3D9 on my Windows 8 machine (testsprite2 runs at 3400 FPS vs 3100 FPS)
This will need tweaking to fix the Windows RT build.

--HG--
rename : src/render/direct3d11/SDL_render_d3d11.cpp => src/render/direct3d11/SDL_render_d3d11.c

include/SDL_config_windows.h

@@ -171,6 +171,9 @@ typedef unsigned int uintptr_t;
 #ifndef SDL_VIDEO_RENDER_D3D
 #define SDL_VIDEO_RENDER_D3D    1
 #endif
+#ifndef SDL_VIDEO_RENDER_D3D11
+#define SDL_VIDEO_RENDER_D3D11	1
+#endif
 
 /* Enable OpenGL support */
 #ifndef SDL_VIDEO_OPENGL