PLAYA-PDF 0.6.1: Regression, refactoring, image extraction

What's Changed

• fix: correct bogus font descriptors with zero metrics by @dhdaines in #134
• feat: extract masks, softmasks, and alternates by @dhdaines in #135
• Good enough JBIG2 support by @dhdaines in #136
• Save Indexed images properly by @dhdaines in #137 (contribution by @lambdalemon)
• fix: avoid writing empty image files by @dhdaines in #139

Full Changelog: v0.6.0...v0.6.1