graphics/mypaint: Update to 2.0.0

Port update port and take maintenership by Greg

Major changes:

• Linear compositing and spectral blending (pigment).
• Layer views.
• Brush strokes dependent on view rotation and view zoom.
• Additional symmetry modes: vertical, vertical+horizontal, rotational, snowflake.
• Expanded flood fill functionality: offset, feather, gap detection and more.
• New brush settings: offsets, gridmap, additional smudge settings, posterize, pigment.
• New brush inputs: barrel rotation, base radius, zoom level, gridmap x/y, direction 360, attack angle.

See full changelog for more details: https://github.com/mypaint/mypaint/blob/master/Changelog.md

PR: 235805
Submitted by: Greg V <greg@unrelenting.technology>