core: window size not restored on leaving .fullscreen to .windowed.

[JohnathanBurchill]

With

const core = @import("mach-core");

and calling within update()

core.setDisplayMode(.fullscreen);
core.setDisplayMode(.windowed);

the window does not return to its original size. The mach-core source Core.zig stores the window size prior to entering fullscreen in last_windowed_size, but restores the size upon entering windowed mode using last_size in self.window.setMonitor(...).