Backends: Fixed various warnings discovered when using MinGW GCC 15/Clang on latest backends.

dx12: 'ImGui_ImplDX12_Data* bd' shadowed local in spite of being in lambda.

backends/imgui_impl_win32.cpp

@@ -571,6 +571,7 @@ ImGuiKey ImGui_ImplWin32_KeyEventToImGuiKey(WPARAM wParam, LPARAM lParam)
     case 51: return ImGuiKey_Comma;
     case 52: return ImGuiKey_Period;
     case 53: return ImGuiKey_Slash;
+    default: break;
     }
 
     return ImGuiKey_None;