Add support for subdivs

[Chad Dombrova]

This is supported by HdStream, so adding this here is a great opportunity to use OpenSubdiv in maya viewport.

In usdview there is a setting for "complexity" which corresponds to subdiv smoothing level, so we'll want an attribute to control that on the HdStream render globals node. see #8.

[Digic Pictures]

Would be nice if the viewport subdiv setting on the mesh shape was used by HdStream (modified by"3" and page up/down hotkeys) and have a global complexity limit on the HdStream render globals node. Switching subdiv levels for the whole scene is a nice feature, but most of the time you'd want to have more granular control.

[Pal Mezei]

This was fixed in #45 , which is essentially a duplicate of this ticket.

[Pal Mezei]

@digicpictures and yes, page up / page down works. :)