Blender Git Loki

Fix T57858: Add validation callback to CustomData layers.

Our mesh validation was only checking cd layout so far, not their actual
data. While this might only be needed for a few types, this is a
required addition for things like imported UVs, else we have no way to
avoid nasty things like NANs & co.

Note that more layer types may need that callback, time will say. For
now added it to some obvious missing cases...