reftests: Add a way to compare different runs

reftests: Add a way to compare different runs

gtk-reftest already had an --output=DIR option to tell it where
to save all the resulting images. Now you can combine this with
the --compare-with=DIR option in a second run to make gtk-reftest
compare the .out.png files from the first run with the .out.png
files of the current run, instead of producing .ref.png files.

The intended use for this is to verify that changes do not affect
the generated output.