Ebiten backend: add missing functions

[gucio321]

• SetShouldClose
• Refresh (noop)
• ...

[0xkalle]

One question, sorry if it makes no sense: Do we need a way to control the TPS here, which we can later use in giu as well to control how often "Update" is called?

[gucio321]

Update = Refresh (some historical stuff in giu)

Generally the backend in cimgui-go is responsible for TPS and Refresh forces another redraw;
This was because in glfwbackend there was some "power saving" system implemented (?) which causes backend does not render if there is no user input (glfw stays in pollEvents) (or at least this is how it used to be before #428). Refresh forced glfw to update by sending an empty event.

In ebiten however I don't see any way to do something similar and ebiten always renders without waiting for events.