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.

configure.in

@@ -2394,6 +2394,7 @@ AC_HELP_STRING([--enable-directx], [use DirectX for Windows audio/video [[defaul
                   , enable_directx=yes)
     if test x$enable_directx = xyes; then
         AC_CHECK_HEADER(d3d9.h, have_d3d=yes)
+        AC_CHECK_HEADER(d3d11_1.h, have_d3d11=yes)
         AC_CHECK_HEADER(ddraw.h, have_ddraw=yes)
         AC_CHECK_HEADER(dsound.h, have_dsound=yes)
         AC_CHECK_HEADER(dinput.h, have_dinput=yes)
@@ -2791,6 +2792,9 @@ AC_HELP_STRING([--enable-render-d3d], [enable the Direct3D render driver [[defau
             if test x$enable_render_d3d = xyes -a x$have_d3d = xyes; then
                 AC_DEFINE(SDL_VIDEO_RENDER_D3D, 1, [ ])
             fi
+            if test x$enable_render_d3d = xyes -a x$have_d3d11 = xyes; then
+                AC_DEFINE(SDL_VIDEO_RENDER_D3D11, 1, [ ])
+            fi
         fi
         # Set up files for the audio library
         if test x$enable_audio = xyes; then