Add windows_with_undefs.h to fix conflicts with windows.h

windows.h adds macros such as `#define GetMessage GetMessageA` which
cause conflicts in naming when for example a class definition is before
windows.h and a use is after windows.h

Add a header that can be used both to include windows.h and remove
defines, and to only remove defines (when windows.h is included by)
external dependencies.

src/backend/d3d12/d3d12_platform.h

@@ -25,4 +25,10 @@
 
 using Microsoft::WRL::ComPtr;
 
+// Remove windows.h macros after d3d12's include of windows.h
+#include "common/Platform.h"
+#if defined(NXT_PLATFORM_WINDOWS)
+#    include "common/windows_with_undefs.h"
+#endif
+
 #endif  // BACKEND_D3D12_D3D12PLATFORM_H_

src/backend/opengl/DeviceGL.h

@@ -19,9 +19,15 @@
 
 #include "backend/Device.h"
 #include "backend/opengl/Forward.h"
+#include "common/Platform.h"
 
 #include "glad/glad.h"
 
+// Remove windows.h macros after glad's include of windows.h
+#if defined(NXT_PLATFORM_WINDOWS)
+#    include "common/windows_with_undefs.h"
+#endif
+
 namespace backend { namespace opengl {
 
     class Device : public DeviceBase {