graphics/mtpaint: update to 3.50.09
- use DISTVERSION instead of PORTVERSION
Changelog-like descriptions in each commit since 3.50.3 [1].
[1] https://github.com/wjaguar/mtPaint/commits/master
Approved by: dbaio, garga (mentors)
Differential Revision: https://reviews.freebsd.org/D30034