d3d12: Import DirectX-Headers, use local d3d12.h in SDL_render.

This avoids lots of build issues with the various D3D12 headers out there (MinGW, old Windows SDKs, etc) and also opens the door for WSL2 libd3d12.so support.

Note that the build system has not been changed; technically _all_ platforms now have d3d12.h but we should only enable the backend when it's actually expected to work.

src/render/direct3d12/SDL_render_d3d12.c

@@ -38,7 +38,18 @@
 #define D3D12_TEXTURE_DATA_PITCH_ALIGNMENT 256
 #endif
 #else
-#include <d3d12.h>
+
+/* From the DirectX-Headers build system:
+ * "MinGW has RPC headers which define old versions, and complain if D3D
+ * headers are included before the RPC headers, since D3D headers were
+ * generated with new MIDL and "require" new RPC headers."
+ */
+#define __REQUIRED_RPCNDR_H_VERSION__ 475
+#ifndef WINAPI_PARTITION_GAMES
+#define WINAPI_PARTITION_GAMES 0
+#endif /* WINAPI_PARTITION_GAMES */
+#include "../../video/directx/d3d12.h"
+
 #include <dxgi1_6.h>
 #include <dxgidebug.h>
 #include <d3d12sdklayers.h>

src/render/direct3d12/SDL_shaders_d3d12.c

@@ -23,7 +23,17 @@
 #if SDL_VIDEO_RENDER_D3D12 && !defined(SDL_PLATFORM_XBOXONE) && !defined(SDL_PLATFORM_XBOXSERIES)
 
 #include "../../core/windows/SDL_windows.h"
-#include <d3d12.h>
+
+/* From the DirectX-Headers build system:
+ * "MinGW has RPC headers which define old versions, and complain if D3D
+ * headers are included before the RPC headers, since D3D headers were
+ * generated with new MIDL and "require" new RPC headers."
+ */
+#define __REQUIRED_RPCNDR_H_VERSION__ 475
+#ifndef WINAPI_PARTITION_GAMES
+#define WINAPI_PARTITION_GAMES 0
+#endif /* WINAPI_PARTITION_GAMES */
+#include "../../video/directx/d3d12.h"
 
 #include "SDL_shaders_d3d12.h"