Blender Git Loki

Fix T42260: By repeatedly joining meshes, you could end up with thousands of UVLayers

This is only an indirect fix, in fact: this commit adds a public API to check
the maximum number of a given layer type (`CustomData_layertype_layers_max()`),
and uses it to forbid too much layer creation in `CustomData_merge()`.

This only affects UVs/VCol data though, but merge behavior in itself is not a bug
actually, how user managed to get thousands of different UV layer names remain
rather mysterious...