This command can be used to compare the rendering of a node
to a reference image. It can also be used to compare the
renderings of two nodes, or to compare two images.
We are likely to use the tool with node files from out testsuite,
which may now refer to custom test fonts, so make them available
in the same way as in the node editor.
If in doubt, you can set GTK_SOURCE_DIR to make the tool find the
fonts.
