Fix [#37380] vertex paint colors don't render.

Another Evil Typo (r) one, you could add much more than the 8 allowed VCol layers!

Note: added some (warning-only) checks in mesh validate functions, but we still have a big issue with new cdlayer merge function, which could generate more than 8 layers of UVs or VCol... Don't know yet how to handle this situation. :(

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@61232 954f8c5b-7b00-dc11-b283-0030488c597c