In GimpSourceCore, when "sample merged" is enabled, derive the
source pickable from the source drawable's image according to the
paint-core's show-all flag manually, instead of using
gimp_paint_core_get_image_pickable(), which uses the destination
image, and would therefore only work when the source and
destination images are the same.

In GimpSourceTool, override GimpPaintTool::paint_prepare() to set
the paint-core's show-all flag according to the source display,
rather than the destination display.