Patch [#29035] Vertex colour baking

There is a new option in the Bake panel to enable baking to vertex colors. Unlike regular baking, this mode does not require a UV map or image to bake to, however the object must have a vertex color layer.

Thanks to:
 - AutoCRC for funding
 - Brech van Lommel and Dalai Felinto for their initial advice on how to implement it
 - Campbell Barton for helping to make this feature work with modifiers and bmesh


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53494 954f8c5b-7b00-dc11-b283-0030488c597c