Add the flag to request a renderer able to render to a texture

Property definitions

sdl2 $ SDLRendererFlags :: targettexture
	# Add the flag to request a renderer able to render to a texture
	fun targettexture: SDLRendererFlags `{ return self | SDL_RENDERER_TARGETTEXTURE; `}
lib/sdl2/sdl2_base.nit:393,2--394,84