External tilesets are disallowed inside multiple contents

[javagl]

The title basically says it all: When using multiple contents that refer to external tilesets, then a message

Error: External tilesets are disallowed inside multiple contents

is printed and the external tilesets are not rendered.

This might be a leftover from the 3DDILES_multiple_contents extension: This explicitly disallowed external tilesets in the contents (see https://github.com/CesiumGS/3d-tiles/pull/583/files ). But in 3D Tiles 1.1, there is no such restriction.

Attached a really minimal test case of one tileset that only has two contents, which in turn are (external) tilesets, to reproduce the issue:

external tilesets in multiple contents.zip