Allocate the memory for a new SDLRect, it must then be freed with free

Property definitions

sdl2 $ SDLRect :: malloc
	# Allocate the memory for a new `SDLRect`, it must then be freed with `free`
	new malloc`{ return malloc(sizeof(SDL_Rect)); `}
lib/sdl2/sdl2_base.nit:477,2--478,49