Real screen framebuffer name

Property definitions

gamnit :: dynamic_resolution $ App :: screen_framebuffer
	# Real screen framebuffer name
	fun screen_framebuffer: Int
	do
		var cache = screen_framebuffer_cache
		if cache != -1 then return cache

		cache = glGetIntegerv(gl_FRAMEBUFFER_BINDING, 0)
		self.screen_framebuffer_cache = cache
		return cache
	end
lib/gamnit/dynamic_resolution.nit:60,2--69,4