Fix texture distortion for flowing liquids (#9561)

Fix texture distortion for flowing liquids (#9561)

Previously textures of the side faces on flowing liquid nodes would
become distorted on different axis depending on the liquid level.  This
is because the nodes always had the same texture coordinates, even when
the generated face could have different sizes.  This solves that problem
by adjusting the texture coordinates for the vertices making up the top
of the faces, so the textures will not look compressed for smaller
faces.