The colormap saves colors as unsigned char, which can be very inaccurate
compared to high precision colors. When adding colors from GimpRGB into
the colormap, use the original value to fill the colormap palette
instead of making a round trip conversion from double to uchar, then
back to double.

This also fixes a direct bug I encountered when adding the current
foreground color in the image colormap. Yet the GimpFgBgEditor or the
GimpColorHistory would still show the color out-of-gamut in cases when
the returned RGB after the roundtrip was not close enough to the
original RGB (even despite using an epsilon in GimpPalette code).