Save fullscreen toggle state to options.txt when F11 is pressed, and restore it on the next launch. When starting in windowed mode, the window now opens maximized instead of at a smaller default size.
Previously, g_isFullscreen was hardcoded to false and never persisted, so the game always started in windowed mode. The window also started at a non-maximized size requiring manual maximization.
Fixes#391
3f1388e06f