A Colorband's CBData color **is not** considered `PROP_COLOR_GAMMA`.
A Brushes color **is** considered `PROP_COLOR_GAMMA`.
(PROP_COLOR_GAMMA is used for colors which would be color managed before
display, could be renamed to something better once...)
This leads to different rgb values in ColorBand.CBData of br->gradient
and brush->rgb for seemingly identical colors. (this is because color
pickers do differently in case block->is_color_gamma_picker/
ui_but_is_color_gamma)
Now it looks like `paint_brush_color_get` is expected to return a color
in sRGB (according to @jbakker this is for legacy reasons) so we need to
run the colorband colors through linear -> sRGB.
It might very well be the case that a much deeper cleanup in this area
is needed, this is just a fix to get gradient brush colors
consistent again...
Maniphest Tasks: T75985
Differential Revision: https://developer.blender.org/D7501
2b094be949