Add test for KHR_node_visibility

[javagl]

An asset for testing the KHR_node_visibility extension.

A first version of this was posted in a comment in the pull request. A version with minor additions was posted in a later comment. It could make sense to add this as a dedicated test asset for the extension here.

The asset uses some nested node structure to display the messages and support status. A summary:

• First row:

  • There is a label that says "The extension is supported"
  • Slightly in front of that, there is a label that says "The extension is not supported"
  • The label in the front has the extension with visible:false. So when the extension is supported, this is not shown.

• Second row:

  • There is a label that says "Does not handle defaults properly"
  • Slightly in front of that, there is a label that says "Does handle defaults properly"
  • The label in the front has visible:undefined, which should default to true. So when the default is handled properly, then this is shown.

• Third row:

  • There are two labels that say "Does not properly handle hierarchy".
  • These are the leaves of a chain of nodes with visbility flags false, false, true and true, true, false, respectively.
  • So they should eventually both not be visible.

(There are some blank rectangles that hide the implementation messages when the extension is not supported at all).

Direct preview to the README body: https://github.com/javagl/glTF-Sample-Assets/blob/add-node-visibility-test/Models/NodeVisibilityTest/README.body.md