* Disabling WindowSizeCallback around fullscreen toggle.

#1294 fixed the issue that toggle fullscreen changes the screen size because of the WindowSizeCallback. The proposed edit by @raysan5 was to comment out WindowSizeCallback which I essentially simplified to disable the callback around the set window monitor functions.

The developers using the ToggleFullscreen function should be aware that they need to size the screen correctly on that same frame. Otherwise they should check if fullscreen and size properly using the GetMonitorWidth and GetMonitorHeight functions.

* Update core.c

Fix issue from merge