GimpPaintCore operates indipendently of a display, and hence needs
to be explictly told when operating in "show all" mode, affecting
the result of paint tools operating in "sample merged" mode.  Add
gimp_paint_core_set_show_all() for that purpose, and call it,
passing the current display's "show all" mode, in GimpPaintTool.
This controls which pickable (the image itself, or its projection)
is used as the sampling source, as per
GimpPaintCore::saved_proj_buffer, and as returned by the new
gimp_paint_core_get_image_pickable() function.