Backends: DX11: Expose vertex constant buffer in ImGui_ImplDX11_RenderState.

DX9 and DX12 users can already alter the projection matrix (in undocumented ways, but possible)

backends/imgui_impl_dx11.h

@@ -21,6 +21,7 @@
 struct ID3D11Device;
 struct ID3D11DeviceContext;
 struct ID3D11SamplerState;
+struct ID3D11Buffer;
 
 // Follow "Getting Started" link and check examples/ folder to learn about using backends!
 IMGUI_IMPL_API bool     ImGui_ImplDX11_Init(ID3D11Device* device, ID3D11DeviceContext* device_context);
@@ -40,6 +41,7 @@ struct ImGui_ImplDX11_RenderState
     ID3D11Device*           Device;
     ID3D11DeviceContext*    DeviceContext;
     ID3D11SamplerState*     SamplerDefault;
+    ID3D11Buffer*           VertexConstantBuffer;
 };
 
 #endif // #ifndef IMGUI_DISABLE