Using multiple shader materials causes them to render on eachother's elements

[NiseVoid]

In this image I have set up two shader materials. One that colors these skill icons, and another that should render this noise texture as the background of an element above it.

Instead of targeting the right quad for the second material (the noise texture one), it instead draws to the first quad using the first material. Removing the elements with the first material makes the second one render to the correct quad.

This issue is similar to sosmething that was fixed before by #242. But in this case it applies only to quads that have a custom material.