app :: App :: maximum_fps
This basically limits how much frame_core is called per second.
Zero (or a negative value) means no limit.
Applications can modify this value even during the main-loop.
	# Limit the frame-rate to a given frequency
	#
	# This basically limits how much `frame_core` is called per second.
	# Zero (or a negative value) means no limit.
	#
	# Applications can modify this value even during the main-loop.
	var maximum_fps = 0.0 is writable
					lib/gamnit/limit_fps.nit:22,2--28,34