Improved thread safety:

• the internal error stack and log file handle are now in a per-thread context object
• JPEG now captures IPTC information in a thread-safe way
• avoid globals where possible for warning capture in libtiff
• use a mutex to avoid re-entering thread-unsafe giflib
• use a mutex to avoid re-entering thread-unsafe tifflib
• use a mutex to avoid re-entering thread-unsafe T1Lib
• use a library handle per thread for freetype 2.
• use an engine handle per thread for freetype 1.x.
• originally these changes broke ABI compatibility, this has been restored.

Other changes:

• clarify the return value of getpixel() RT #81198

• fixed a race condition in parallel testing for T1

• fixed a bug in handling yoff for untransformed image-based fills

• documentation improvements for Imager::Fill

• FT2: report the library version while testing.