Source Texture defines the texture that is cloned. Parts of
it are copied over to the target texture while you are drawing.
Hold down the ALT key to have the
cursor change into the clone cursor
After that click on the texture that you want to be cloned. Release
the ALT key and click on the target texture. The Offset
values will be adjusted so that the clicked position on the source
texture is copied to the area where you clicked on the target