Merge misc changes from docking branch to reduce small drift.

In particular:
- imgui.cpp : move UpdateInputEvents() higher in NewFrame() to match docking + update RenderMouseCursor() to match.
- imgui_draw.cpp: ImDrawList::_ResetForNewFrame() change from c807192ab
- Backends: SDL2. Add MouseWindowID + change SDL_CaptureMouse() test to match docking branch. Not strictly necessary but aimed at reducing drift because we go on and fork this file.
+ moved responsability of checking valid names to TabBarGetTabName() to simplify both branches.

backends/imgui_impl_allegro5.cpp

@@ -275,7 +275,7 @@ void ImGui_ImplAllegro5_InvalidateDeviceObjects()
     ImGui_ImplAllegro5_Data* bd = ImGui_ImplAllegro5_GetBackendData();
     if (bd->Texture)
     {
-        io.Fonts->SetTexID(nullptr);
+        io.Fonts->SetTexID(0);
         al_destroy_bitmap(bd->Texture);
         bd->Texture = nullptr;
     }