The compositor execute functions have a `rendering` argument to specify
if the compositor is executing as part of the render pipeline. But the
render context argument is null if we are not rendering, so the
`rendering` arguement is redundant and can be removed.
Additionally, we no longer use use_file_output as a hack to detect
rendering.
Pull Request: https://projects.blender.org/blender/blender/pulls/120659
757da9dbc1