-void gis_viewer_render_tile (GisViewer *viewer, GisTile *tile);
-void gis_viewer_render_tiles(GisViewer *viewer, GisTile *root);
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-void gis_viewer_begin(GisViewer *viewer);
-void gis_viewer_end (GisViewer *viewer);
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-gpointer gis_viewer_add(GisViewer *self, GisObject *object,