Don't force the instantiators on
Use the VTK_RENDERING_BACKEND variable for deps

This makes it easy to switch between OpenGL legacy, and the new
OpenGL backend without hardwiring one or the other. It does require
a recent enough VTK that this variable is set/exposed in the Config.
Merge pull request #19 from cryos/opengl2-build-updates

Opengl2 build updates