In the Bucket Fill tool, when using line-art mode, use a
GimpImageProxy as input for the GimpLineArt object, instead of
using the image direclty, when the line-art source is "all visible
layers".  Set the proxy's show-all mode according to the active
shell, so that the line-art uses the full image projection,
including out-of-canvas regions, when in show-all mode.