"Zoom" for windowed mode

[stuartlangridge]

The windowed mode for syzygy is rather small on modern large monitors. It would be useful if there were a 2x or even 3x "zoom" for the window; the pixel-art nature of it would I think be fine with this (so each pixel becomes a 2x2 square of pixels in "2x zoom" mode). I think that there are relatively simple SDL features to enable this: SDL_RenderSetLogicalSize might be the way, although I am no expert.

(It might also be helpful to document explicitly that one needs --fullscreen=false to run in windowed mode.)