Amend sxbp to be type-compatible with v0.26.0 of libsxbp

The latter is now 32-bit compatible

remove mentions of libsxbp being agpl-licensed

this is no longer the case

Update for renamed dependency

The dependency libsaxbospiral has been renamed to libsxbp.
This release amends sxbp appropriately to handle this

Improve error reporting when PNG output requested when unsupported

Amend for libsaxbospiral v0.22.0

This was necessary because the header files of v0.21.x are incompatible with the binary ABI of v0.22.0

re-organise order of command-line options

String input

Add -S/--string argument for specifying input data as string on the command line instead of as file

Re-instate the ability to render images to PNG format

PBM format is the default

Improve functional test

Minor updates to do with functional testing
Test input is now a static value so that the test file should always stay the same between versions.

Refactor for libsaxbospiral v0.21

- Swapped PNG output for PBM for now (this is easier to test)
- Also improved display of command-line options glossary