Fix #35289: UV layout export to image was extremely slow for large meshes. This
was due to slow implementation of slice operation for things like mesh uv data.
Made that faster now for cases where the internal storage is an array.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56674 954f8c5b-7b00-dc11-b283-0030488c597c