Version 1.3.0: Triangle Sets and Mirroring

This is a minor release of the Core Specification of 3MF.

It clarifies several ambiguities, resolves inconsistencies and adds new concepts.
New Concepts compared to 1.3.0:

• trianglesets allow to encode sets of triangles into 3MF-files. Applications can apply grouping operations and assign properties to a list of triangles.
  Figure: Triangle sets that correspond to engineering surfaces of a part. One of the triangesets is rendered differently to indicate selection.

• Mirroring of Meshes: the mirrormesh-element allows to explicitly encode the relation of one mesh-object to be a mirrored instance of a different mesh in the 3MF file.
  Figure: a mesh, the mirror plane and its mirrored copy
  

• reccomendedextensions is a list of extension namespace prefixes that allow to encode which extensions are recommended for a consumer to read a file.

Clarifications compared to v1.2.3:

• the interpretation of transformations with negative determinants has been clarified
• the usage of the ZIP-format and the zip64 extensions has been clarified
• the notion of object-level and package-level thumbnails has been clarified

Version 1.2.3: Clarification of content types

Changes compared to version 1.2.2:

• Added clarification that content types are not restricted to the list in Appendix C.1.

First public Markdown release

This is the first public release of the 3MF Core specification in Markdown. It has been reformatted from the original documents and updated with relatively minor changes from 1.2.1.