This change fixes an issue where ImGuiHelper would crash on destruction if you created more than one instance at the same time.
This crash occurred when ImGui::DestroyContext() was called, because when the second instance was destroyed there was no current context causing it to crash.
This fixes it by making ImGuiHelper store and manage it's own context.
Having two instances of ImGuiHelper is useful in cases where you want to use imgui with multiple views.
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