Diffuse color when covered by a light source

The RGB values should be premultiplied by the alpha value.

Property definitions

gamnit $ SmoothMaterial :: diffuse_color
	# Diffuse color when covered by a light source
	#
	# The RGB values should be premultiplied by the alpha value.
	var diffuse_color: Array[Float] is writable
lib/gamnit/depth/more_materials.nit:40,2--43,44