Refactoring Ptex Mipmap and Analytic Displacement code

Moving Takahito's implementation into the core API:
- added <gl/d3d11>PtexCommon.<glsl/hlsl> shader code
- added control to enable Ptex common trunk in <gl/d3d11>DrawRegistryBase classes
- fixed GL & D3D11 ptexViewer examples to use the new API