Is shadow_context.depth_texture ready to be used?

Property definitions

gamnit :: shadow $ App :: shadow_depth_texture_available
	# Is `shadow_context.depth_texture` ready to be used?
	fun shadow_depth_texture_available: Bool
	do return supports_shadows and shadow_context.depth_texture != -1
lib/gamnit/depth/shadow.nit:47,2--49,66