MultiMonitor Support

[DBHeise]

I'm using ebiten to write a screen saver, however I have multiple setups that have multiple monitors in different configurations. I have one windows box with 4 identical monitors in a grid pattern, and a mac with 3 monitors in an unusual pattern.

I was thinking that in order to have a screen that covers all monitors, I need to set the "window" to the top left most monitor and then size it to cover all other monitors. Yes in some configurations this will mean there are areas of my "window" that are not visible in any display, but for what I'm doing that's ok.

Another possible option is to create a "game" for each monitor, but that would be tedious and harder to manage interactions between monitors.

I see that the ui.Monitor struct has a boundsInGLFWPixels field that contains the information I need to do my calculations (assuming my logic will in-fact work), so I propose that that field be exposed, perhaps by a Postion function that simply returns that field.

If this is acceptable, I already have the code changed and can make a PR.

If there's another way to accomplish drawing to multiple monitors, I'm all ears.

[DBHeise]

I believe this would solve #1114 as Size() is already implemented in the code.

[hajimehoshi]

Interesting, but your purpose sounds pretty niche and we should discuss more whether we can add a new API for this or not