Backends: DX12: let bd->FrameIndex start at 0 to match docking. (#8961)

backends/imgui_impl_dx12.cpp

@@ -106,7 +106,7 @@ struct ImGui_ImplDX12_Data
 
     bool                        LegacySingleDescriptorUsed;
 
-    ImGui_ImplDX12_Data()       { memset((void*)this, 0, sizeof(*this)); frameIndex = UINT_MAX; }
+    ImGui_ImplDX12_Data()       { memset((void*)this, 0, sizeof(*this)); }
 };
 
 // Backend data stored in io.BackendRendererUserData to allow support for multiple Dear ImGui contexts