G'day y'all.

My issue is this: I have a stairway in a map of mine and a texture that I want to use on the walls. The problem is that the wall texture doesn't tile vertically, so instead, I want to shear the texture across a rhombus-shaped brush that will be the wall of the stairway. I've seen this done in the last stage of pl_goldrush, so my question is: How is this done?

Thank all yee kindly.