IO: set io.KeyModsPrev in EndFrame() to tolerate backend writing to io.KeyMods without breaking. (#4439)

Amend ff428f1

imgui.cpp

@@ -4049,7 +4049,6 @@ void ImGui::NewFrame()
 
     // Update keyboard input state
     // Synchronize io.KeyMods with individual modifiers io.KeyXXX bools
-    g.IO.KeyModsPrev = g.IO.KeyMods;
     g.IO.KeyMods = GetMergedKeyModFlags();
     memcpy(g.IO.KeysDownDurationPrev, g.IO.KeysDownDuration, sizeof(g.IO.KeysDownDuration));
     for (int i = 0; i < IM_ARRAYSIZE(g.IO.KeysDown); i++)
@@ -4487,6 +4486,7 @@ void ImGui::EndFrame()
     // Clear Input data for next frame
     g.IO.MouseWheel = g.IO.MouseWheelH = 0.0f;
     g.IO.InputQueueCharacters.resize(0);
+    g.IO.KeyModsPrev = g.IO.KeyMods; // doing it here is better than in NewFrame() as we'll tolerate backend writing to KeyMods. If we want to firmly disallow it we should detect it.
     memset(g.IO.NavInputs, 0, sizeof(g.IO.NavInputs));
 
     CallContextHooks(&g, ImGuiContextHookType_EndFramePost);