graphics/mtpaint: update to 3.50.08

• remove USES=tar:bzip2 related to 3.40 distfile tar.bz2
• use ${PREFIX}/share/man for manual pages
• turn on WEBP option by default (forgotten in last update)

Changelog-like descriptions in each commit since 3.50.3 [1].

[1] https://github.com/wjaguar/mtPaint/commits/master

Differential Revision: https://reviews.freebsd.org/D29321