graphics/mtpaint: Update to 3.50.03

Changelog-like descriptions in each commit since 3.40 [1].

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

PR: 252926
Submitted by: Nuno Teixeira <ed.arrakis@gmail.com> (new maintainer)
Approved by: arrowd (mentor)
Differential Revision: https://reviews.freebsd.org/D28336