This provides options for hardware and software rendering with the
gl API and updates the vtk package to make use of those options:
* Create new "gl" virtual package, provided by mesa
* Add external-only "opengl" package, which represents a system
install that provides gl
* For vtk: prefer system gl implementation by default to get speed
up from hardware rendering. When software rendering is specifically
requested (+osmesa), try to use the llvmpipe approach, as it is
much faster.
* Make qt dependency optional for vtk
* Add basic support for VTK on osx
* qt: Depend on virtual gl package, rather than mesa impl
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