COMP: Revisit initialization of Autoscoper_RENDERING_BACKEND

Revisit approach originally introduced in 6912ab0 (ENH: Enable CUDA on
Windows and Linux) 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.

SuperBuild/External_Autoscoper.cmake

@@ -57,16 +57,28 @@ if(NOT DEFINED ${proj}_DIR AND NOT ${SUPERBUILD_TOPLEVEL_PROJECT}_USE_SYSTEM_${p
       )
   endif()
 
-  # set(Autoscoper_OPENCL_USE_ICD_LOADER TRUE)
-  # if(APPLE)
-  #   set(Autoscoper_OPENCL_USE_ICD_LOADER FALSE)
-  # endif()
-
-  set(Autoscoper_OPENCL_USE_ICD_LOADER FALSE)
-  if(APPLE)
+  set(_msg "")
+  if(DEFINED ENV{Autoscoper_RENDERING_BACKEND})
+    set(_msg " (initialized from env. variable)")
+    set(Autoscoper_RENDERING_BACKEND $ENV{Autoscoper_RENDERING_BACKEND})
+  elseif(NOT DEFINED Autoscoper_RENDERING_BACKEND)
+    set(_msg " (default value)")
     set(Autoscoper_RENDERING_BACKEND "OpenCL")
+  endif()
+
+  ExternalProject_Message(${proj} "Setting Autoscoper_RENDERING_BACKEND to ${Autoscoper_RENDERING_BACKEND}${_msg}")
+
+  if(NOT "${Autoscoper_RENDERING_BACKEND}" MATCHES "^(CUDA|OpenCL)$")
+    message(FATAL_ERROR "Autoscoper_RENDERING_BACKEND variable is expected to be set to CUDA or OpenCL")
+  endif()
+
+  if(Autoscoper_RENDERING_BACKEND STREQUAL "OpenCL")
+    set(Autoscoper_OPENCL_USE_ICD_LOADER TRUE)
+    if(APPLE)
+      set(Autoscoper_OPENCL_USE_ICD_LOADER FALSE)
+    endif()
   else()
-    set(Autoscoper_RENDERING_BACKEND "CUDA")
+    set(Autoscoper_OPENCL_USE_ICD_LOADER FALSE)
   endif()
 
   if(UNIX AND NOT APPLE)