First stab at adding genete's suggested 'Curve Warp' layer. It's pretty slow at...

First stab at adding genete's suggested 'Curve Warp' layer.  It's pretty slow at the moment, because it uses get_color() to render the context pixel by pixel rather than using accelerated_render() to render a block at once.

git-svn-id: https://synfig.svn.sourceforge.net/svnroot/synfig@2164 1f10aa63-cdf2-0310-b900-c93c546f37ac