Add support for imposters for molecular rendering

Add in a new directory ChemistryOpenGL that includes a factory
override for vtkMoleculeMapper that uses imposters to draw the
spheres and cylinders. Supports picking.

Change-Id: I718cc4fbae25c3dc7ab82bf19ed7a12e472c51b2
Make it so that fragment shaders can modify vertexVC

Somefragment shaders want to modify vertexVC which previously
was a hard coded varying that is used by the lighting calcs.
Made it no longer hard coded. Fixes imposters for Intel chips.

Change-Id: I3feae834a0b6c9a7eb775404b083bdaba55dfb49
Merge topic 'mol_mapper' into master

5b62e02d Make it so that fragment shaders can modify vertexVC
6207dbb1 Add support for imposters for molecular rendering