COMP: Revisit initialization of Autoscoper_RENDERING_BACKEND

[jcfr]

Revisit approach originally introduced in 6912ab0 (ENH: Enable CUDA on Windows and Linux) through #64 to support initializing Autoscoper_RENDERING_BACKEND based on env. variable of the same name.

This will allow to enable the CUDA rendering backend only in build environment where CUDA is available.

[jcfr]

Example of output obtained from an existing build tree:

$ cmake .
[...]
-- Setting Autoscoper_RENDERING_BACKEND to OpenCL (initialized from default value)
-- SuperBuild - First pass
-- SuperBuild - First pass - done
[...]
-- Configuring done
-- Generating done

$ Autoscoper_RENDERING_BACKEND=CUDA cmake -UAutoscoper_RENDERING_BACKEND .
[...]
-- Setting Autoscoper_RENDERING_BACKEND to CUDA (initialized from env. variable)
-- SuperBuild - First pass
-- SuperBuild - First pass - done
[...]
-- Configuring done
-- Generating done

$ Autoscoper_RENDERING_BACKEND=OpenCL cmake -UAutoscoper_RENDERING_BACKEND .
[...]
-- Setting Autoscoper_RENDERING_BACKEND to OpenCL (initialized from env. variable)
-- SuperBuild - First pass
-- SuperBuild - First pass - done
[...]
-- Configuring done
-- Generating done

$ cmake -UAutoscoper_RENDERING_BACKEND .
[...]
-- Setting Autoscoper_RENDERING_BACKEND to OpenCL (initialized from default value)
-- SuperBuild - First pass
-- SuperBuild - First pass - done
[...]
-- Configuring done
-- Generating done