Remove global variables

[cxong]

No description provided.

[cxong]

An intermediate step is to keep the global variables, but:

• name them with a g* prefix so they are easily visible
• pass them through parameters instead of having different functions reference them
• try to get to the stage where only one module or function ever uses the global directly

This means it'll be easy to simplify the lifetime management of the variables, and eventually combine them into local structures. This was done with the credits loading/displaying system recently.

[cxong]

To search for global variables search for \bg[A-Z]\w*

• \b - word boundary
• g - g
• [A-Z] - an uppercase letter
• \w* - rest of the word

[cxong]

Closing because this is a huge task that will take a long time to complete; treat it more as a style or refactor guide.