libmypaint: Update to 1.6.1

Changelog:
1.6.1 - libtool '-release' flag no longer used

1.6.0
This minor version release includes:

    The new mypaint_brush_stroke_to_2_linearsRGB function.
    Some performance improvements, particularly improved auto-vectorization in the spectral->rgb conversion function (thanks to SleepProgger).
    Improved documentation (still lacking a comprehensive overview).
    New translations for Croatian, Portuguese, Valencian, English (UK) and Dutch

(ryoon)