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)

This leads to different values in ColorBand.CBData.rgba of br->gradient
and brush->rgb for seemingly identical colors.(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? If this is the case, 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 bandaid to get gradient brush colors
consistent again...