SDL2 window initialization flags

The default value supports OpenGL and window resizing.

Property definitions

gamnit :: display_linux $ GamnitDisplay :: sdl_window_flags=
	# SDL2 window initialization flags
	#
	# The default value supports OpenGL and window resizing.
	var sdl_window_flags: SDLWindowFlags = (new SDLWindowFlags).opengl.resizable is lazy, writable
lib/gamnit/display_linux.nit:104,2--107,95