Simplify the code by using colorconv_synfig2gdk() rather than repeating its functiona...

Simplify the code by using colorconv_synfig2gdk() rather than repeating its functionality here.  Incidentally, the last change fixed a bug illustrated here: wiki.synfig.com/images/d/d1/Color-editor-linear-bug.jpg -- the positions of the three R,G,B sliders should all be pointing at the same color, but they weren't.  The lower image shows how it looks after the previous commit - all three slider positions are the same color.

git-svn-id: http://svn.voria.com/code@754 1f10aa63-cdf2-0310-b900-c93c546f37ac