Fix tonemap CLI

Author: dvicini

src/python/python/tonemap.py

@@ -2,7 +2,8 @@
     import sys, os
     import argparse
     from concurrent.futures import ThreadPoolExecutor
-    import mitsuba.scalar_rgb as mi
+    import mitsuba as mi
+    mi.set_variant('scalar_rgb')
 
     mi.set_log_level(mi.LogLevel.Info)
     te = mi.ThreadEnvironment()