Implement global GUI scale option independent of window size

[bunabyt3]

Intended to fix #1456.

Scaling is no longer based on the size of the game window. Now, there's a "global scale" value that the user can change. 1.0 represents the real size of the GUI graphics, while 2.0 is the default, matching classic Minecraft behavior.

I still haven't figured out how to change the size of the menu or status text. I don't think this will be complete until that's implemented. Additionally, I think it would be better if the GUI had an automatic scaling option which could select the best global scale value for the user's screen resolution. I'm just not sure how to implement these features yet.