Blender Git Loki

Cleanup/Docs: Add comments to Mesh header, rearrange fields

Most of the fields in Mesh had no comments, or outdated misleading
comments. For example, "BMESH ONLY" referred to the BMesh project,
not the data structure. Given how much these structs are used, it should
save a lot of time to have proper comments.

I also rearranged the fields in mesh to have a more logical order. Now
the most important fields come first. In the process I was able to
remove 19 bytes of unnecessary padding (31->12). I just had to
change a `short` flag to `char`.

Differential Revision: https://developer.blender.org/D13454