Fix crash when window is minimized

[JujuAdams]

When minimizing the game window, GameMaker has a habit of returning the window width and height as 0 pixels. This causes a knock-on crash in __Update() when trying to create a surface with 0 width and 0 height. Further, __Render() crashes in all cases if the internal ImGui_GM rendering surface has disappeared for whatever reason.

This PR addresses both issues and also provides warnings in the output console every 5 seconds. These warnings may be extraneous.