Fixes on several bake modes

• New Material ID baking: assign a unique, distinct color to every material, instead of using the material slot number as color index. Limitations: Support for up to 256 materials; due to the list being persistent only during the current session, between sessions (close and open de file) the result would be different if the materials with users in the scene change (creations, deletions, renames).
• Vertex Colors layers present in the objects were overwritten in certain bake modes (Material ID, Element ID, Cavity, Dust, Paint Base, Selection), with the side effect of making some of these modes don't work themselves, generating straight black images.
• Roughness and Smoothness were not delivering correct results due to the objects materials being replaced by void materials before the bake would start.
• Diffuse, Roughness and Smoothness weren't working properly in multi-object bakes, materials other than the active object active material slot weren't baking.
• Now, when baking Diffuse, Roughness and Smoothness, support image nodes created during baking are deleted once the bake ends.
• Roughness and Smoothness baked images backgrounds set both to black: no more white background for Smoothness.
• Some keywords used on object names during automatic detection of bake pairs (selected to active baking) were producing some false detections due to being too generic, so they have been deleted from the lists used for looking for matches. The letters that must not be used as acronyms from now on, for bakes of the like "high to low" not to fail, are: 'l' for lowpoly, 'h' for highpoly, 'c' for cage, 'f' for float