Compositor: Move Execution Mode and Precision from bNodeTree to Scene

This allows to expose these settings in the Performance panel in the
render buttons. Also moves compositor-specific options away from the
generic node tree structure.

For the backwards-compatibility the options are still present in the
DNA for the bNodeTree. This is to minimize the impact on the Studio
which has used the GPU compositor for a while now. They can be
removed in a future release.

There is no functional changes expected.

Pull Request: https://projects.blender.org/blender/blender/pulls/121583

tests/python/compositor_realtime_render_tests.py

@@ -17,7 +17,7 @@ except ImportError:
 
 ENABLE_REALTIME_COMPOSITOR_SCRIPT = "import bpy; " \
     "bpy.context.preferences.experimental.use_experimental_compositors = True; " \
-    "bpy.data.scenes[0].node_tree.execution_mode = 'GPU'"
+    "bpy.data.scenes[0].render.compositor_device = 'GPU'"
 
 
 def get_arguments(filepath, output_filepath):