Upon release of the new mupdf and pymupdf, flip on JBIG2 by default

[MerlijnWajer]

Currently the default mask encoding is not JBIG2, since mupdf 1.18 has bugs dealing with it:

• https://git.ghostscript.com/?p=mupdf.git;a=commit;h=0f7fd8a26b576e80f626dcd76103eabd1ebc7c83
• https://git.ghostscript.com/?p=mupdf.git;a=commit;h=a5864fcb95ef7a8a492f6b1dd5c85d5be520a2c0
• https://git.ghostscript.com/?p=mupdf.git;a=commit;h=a7e4c904b2a42a3ee79b82db49233f14208689ca