cbz: Use csize and usize fields from central directory.

Don't rely on the csize and usize fileds being set in the individual
entry headers.
Allow NULL callback functions in the mesh processor.

Let either or both of the 'prepare' and 'process' callbacks be no-ops.
Revise test-device; thresholding and exhaustive checking.

The original version of the test-device could characterise pages
as being grayscale/color purely based on the colorspaces used.

This could easily be upset by grayscale images or shadings that
happened to be specified in non-grayscale colorspaces however.

We now look at the actual shading and image color values, and use
a threshold value to allow for some measure of rounding errors in
color values that are in practice grayscale.