C.5 Release 0.12b2

New and changed functionality:

• Add `unit' special, and fix bugs in the `mark' and `strut' specials. This causes a few single-pixel changes in the reported locations of marks and the bounding-boxes coerced by struts.
• Conversions between units (`pt', `cm', and so on) are generalised, and extra library functionality means you'll get this right more often.
• Refactoring of internal logic handling magnification and unit conversions. This is tremendously error-prone, and difficult to write a regression test for, but it produces apparently correct results as far as I've been able to test it. Be on the lookout for subtle errors.
• Various uninitialised-variable bugs caught.