Refactor Smooth Mesh display

[sirpalee]

At the moment we are controlling smooth-mesh display via changing of the subdivision method when returning the HdMeshTopology. Since that's going to be broken when subdiv levels are previewed, we need to look into alternatives.

I think we could get the same results by setting the subdivision style to Catmull-Clark and setting the enable flat shading flag on the display style.

[sirpalee]

I tried using HdDisplayStyle's enableFlatShading and meshes with a Catmull-Clark subdivision setting, but it doesn't work. For now I'm going to stick to the current approach that works, and limit this behavior to HdStreamRenderDelegate.

Removing this ticket from Beta 1 as it requires a bit more digging around and the feature already works.