Added GameWindowFlags.FixedWindow to allow for fixed border SDL Windows

Setting GameWindow.WindowBorder to WindowBorder.Fixed does nothing and results in the message "SDL2 cannot change to fixed-size windows at runtime.". Since the WindowFlags.RESIZABLE flag was hardcoded it was impossible to change. I have added GameWindowFlags.FixedWindow which can be passed to the GameWindow constructor and will prevent the RESIZABLE flag from being set when creating the SDL window. Should be backwards compatible