This is a common dynamics, for instance when you do colorization bases.
This allows to control the size of the brush with pressure and always
keep unified color without opacity changes.
This patch adds a Dynamic for the 'Color From Gradient' option
and three Paint tool presets (Airbrush, Paintbrush, Pencil), which
use this preset.
new file: data/dynamics/Color-From-Gradient.gdyn
new file: data/tool-presets/Paint/Airbrush-Color-From-Gradient.gtp
new file: data/tool-presets/Paint/Paintbrush-Color-From-Gradient.gtp
new file: data/tool-presets/Paint/Pencil-Color-From-Gradient.gtp
modified: data/dynamics/Makefile.am
modified: data/tool-presets/Paint/Makefile.am
modified: AUTHORS
modified: authors.xml
(cherry picked from commit 54b28a5b60)