Use aspect ratio from monitor instead of hard-coding 16/9

[8bitslime]

As the title implies, instead of a macro defining a constant 16 by 9 ratio, use the user's monitor aspect ratio for rendering. Perhaps the current monitor that the game window is on, or just the primary monitor, whichever feels more natural.

[rexim]

@8bitslime it's a very good idea! Thank you submitting an issue for that!