The problem is with mipmapping, which is not full white at the lowest level(s) but grey. You can either create the mipmap chain yourself and make sure the lowest level is a white pixel or simply disable mipmapping for the shadow texture:
The problem is with mipmapping, which is not full white at the lowest level(s) but grey. You can either create the mipmap chain yourself and make sure the lowest level is a white pixel or simply disable mipmapping for the shadow texture: