fix [#33797] decimate modifier bug on uv-coordinates when mesh uses vertex color

if vertex colors had no seams - it would interpolate the UV's too, now interpolate per-layer.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53660 954f8c5b-7b00-dc11-b283-0030488c597c