This reverts commit ff918df889.
This changed it from "RendererHasFocus" to "UIHasFocus", which is
wrong. Specifically, it broke for non-Render to Main Window cases where
the renderer window isn't managed by wx. It also broke the pending
exclusive fullscreen support, which checks this function to determine if
the renderer is on top so it can full-screen it.
We'll add a new hook, "UIHasFocus", in the next commit.
ee087f5953