Save the rendering backend used to the config file

This detail is important for projects using VTK, it will also be used
to populate the VTK_LIBRARIES variable with implementations for any
selected modules.

Change-Id: I4f15f9bb221061ab369b5970200617b2dea66b50
Merge topic 'configure-backend' into master

0e8c8e14 Save the rendering backend used to the config file