Fix ImGui sometimes not respecting the viewport resolution in-Editor

ImguiAtomSystemComponent isn't guaranteed to initialize before ImGuiManager, which caused some issues.
Additionally, because we allow the user to configure ImGui's render resolution, I've refactored the window size override into a new OverrideRenderWindowSize API in ImGuiManager to decouple it from the target render resolution.

Gems/AtomLyIntegration/ImguiAtom/Code/Source/ImguiAtomSystemComponent.h

@@ -48,6 +48,7 @@ namespace AZ
             void Deactivate() override;
 
         private:
+            void InitializeViewportSizeIfNeeded();
 
             // OtherActiveImGuiRequestBus overrides ...
             void RenderImGuiBuffers(const ImDrawData& drawData) override;
@@ -57,6 +58,7 @@ namespace AZ
             void OnViewportSizeChanged(AzFramework::WindowSize size) override;
 
             DebugConsole m_debugConsole;
+            bool m_initialized = false;
         };
     } // namespace LYIntegration
 } // namespace AZ