SImGuiOverlay changes to support Global Invalidation

[avanbrocklinriot]

Sets the SImGuiOverlay widget as volatile, preventing the Invalidation system from caching its Paint data and ultimately forcing OnPaint to be called every frame.

[avanbrocklinriot]

The PR could also just add Invalidate to the SetDrawData if that is preferred.

void SImGuiOverlay::SetDrawData(const ImDrawData* InDrawData)
{
	DrawData = FImGuiDrawData(InDrawData);
	Invalidate(EInvalidateWidgetReason::Paint);
}