Users almost always need a viewer node, so add one to the default node
tree.
Note that the backdrop is already enabled by default, so the backdrop
image will be visible as soon as a render is available (no need to
manually add viewer node or click `cltr+shift+LMB` to show a backdrop
image.
Pull Request: https://projects.blender.org/blender/blender/pulls/138890
df6e65dd93