Previously, the param2-controlled horizontal/vertical merge feature
(which was undocumented and forgotten) was always active, causing
uses of param2 other than "glasslikeliquidlevel" to affect H/V
merging.
Only respect H/V merge bits when paramtype2 = "glasslikeliquidlevel".
H/V merge bits and liquid level bits are designed to be used
simultaneously.
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.
