Add Terrain351 shader

[BlackYps]

This is the same as Terrain301, it just uses the full mask range.
I also disabled the clamping of slider values. You can now input arbitrary values in the text box above a slider. This is important for things like the light multiplier that needs to be bigger than 2.1 to trigger the advanced water shader.
Before this change, if you loaded such a map and clicked on the light tab, the value would silently be clamped to 2.0