UI Toolkit meshes are fully rendered and rasterized when opacity is 0

Steps to reproduce:

1. Load RenderDoc
2. Open the Package Manager
3. With the UI Debugger, set the opacity of the package list to 0
4. Take a RenderDoc capture
5. Observe the draw calls with the mesh overlay

Actual results: 

All mehses are drawn

Expected results: 

No mesh is drawn for the package list