Add a NineSlice from Mesh class, to create scalable textures

[PaologGithub]

A NineSlice texture is to cut the texture in nine, like this: 4 Corners 4 Borders 1 Middle

With this, you can use one 32x32 texture, to make buttons that have the same texture, but not streched or squeeshed.

This pull request add a new NineSlice class, that is based from the Mesh class. It adds too a nine_sliced texture, so we can see the nine cuts.