giflib 4.2.0 released

First giflib release since I reassumed the lead. Short version: lots of useless old cruft thrown out, everything Coverity-scanned, one minor resource leak found and fixed.

As I’ve previously noted, this code was in astonishingly good shape considering its great age. I vigorously beat the dust out of it with Coverity and cppcheck, but found only one very minor bug that way – a malloc leak following a malloc failure in the code that makes color-table structures. I think it is rather likely this case has never actually been triggered.

I retired six utilities, added a bunch of documentation and made it HTML-able, fixed a minor bug in how output GIF versions are computed in an upward-compatible way, and fixed a thread-safety problem. I added a rudimentary regression-test suite; this could use some more work. All tracker bugs have been resolved and closed.

Next release, 5.0, will make one very minor change in the API near extension blocks.