Backends: clear bits set in io.BackendFlags on backend Shutdown(). Clear BackendPlatformName. (#6334, #6335)

Amended with fix for missing clear for ImGuiBackendFlags_HasGamepad.

backends/imgui_impl_dx10.cpp

@@ -567,6 +567,7 @@ void ImGui_ImplDX10_Shutdown()
     if (bd->pd3dDevice) { bd->pd3dDevice->Release(); }
     io.BackendRendererName = nullptr;
     io.BackendRendererUserData = nullptr;
+    io.BackendFlags &= ~ImGuiBackendFlags_RendererHasVtxOffset;
     IM_DELETE(bd);
 }