modules/compose: set default layers

In case some code path creates module contexts and renders without
applying settings, it's important to ensure there are defaults.

As-is this would have crashed compose because compose_layers would
have been NULL, and compose_create_context() assumed compose_layers
always contained something useful.

Montage would have been an example of this, though for other reasons
montage has had compose disabled so I don't think anything currently
would have triggered this.

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