Save fullscreen state on game close

[isrichards6]

When reopening the game you need to manually toggle fullscreen each time you play. This makes situations where you're using only a controller tedious, for example using Steam Big Picture mode.

My proposal is to save the sate of the fullscreen toggle similarly to how it's saved in abe2.ini for stretch and smoothing. This is separate from #1672 in that it's solely interested in keeping settings state rather than adding command line options.

Edit: After looking at the behavior of the game it appears that toggling smoothing and stretching doesn't save to abe2.ini but it does read from it if you update it manually.

[isrichards6]

I'm still getting familiar with the codebase but here's what I believe could work as a solution.

Suggested implementation:

1. Create global variable to keep in Input.cpp to keep track of users desired fullscreen mode
2. Add new function for updating global variable
3. Add additional entry in gCustomSaveEntries found in Input.cpp using global variable.
4. Call SDL_SetWindowFullscreen during the execution of NewParseSettingsIni()
5. When user toggles f12 in sys.cpp also call function to update preference variable