[WIP] Drag-resizable game window

Okay, so after quite a lot of time spent banging my head against the wall that is SDL rendering functions, I seem to have broken through with this.

There are still numerous issues, and probably a memory leak or three somewhere, but so far it seems to work without terrible problems. Works in fullscreen, even.

Right now this is all for SDL ASCII mode. I didn't try to avoid getting the curses side involved yet, and I'm pretty sure it won't work at all well with graphical tiles, but as a first step this is good enough. I only really found this issue because I was idly curious as to what issues had bounties on them, and #933 seemed like it was the least problematic to address. :P

TODO:
- Fix framebuffer artifacts after resizing *properly*.
- Make sure the curses build is not affected
- Make graphics tiles/tilecontext work with this as well
- Save and reuse window size
- Make toggle_fullscreen actually work ingame (highly doubtful)
- Add window size controls to options alongside terminal size
- Add font selector to options
- Add font preview to font selector when/if that exists