cairo_surface_show_page() is only available from Cairo 1.6.0 onwards, but cairo_show_page() is just as easy
and is available from 1.2.0.

If there are large changes in intensity between shells, the R-factor should reflect it.
The old formulation did not have that property.  Weak resolution shells would show up
with low R-factors because they contributed very little to the overall R-factor, but
the actual correspondence between intensities could still be very poor.

Conflicts:
	libcrystfel/src/peaks.c
	src/indexamajig.c

This means that the wavelength gets put into the HDF5 file.