Skip the mesh joint block as the format specifies

Joint records sit between the level of detail data and the morph target
entries, 136 bytes each, and the reader stepped over none of them: it
read the count, called it unused, and read the target entries out of
joint data.

Qt Quick 3D never writes joints, so nothing it produced is affected, but
the block is part of the layout and Qt 3D Studio wrote it.

Change-Id: Ia33e3661e6e3e903cf51c8ace31ee7c6a58d7a8f
Reviewed-by: Christian Strømme <christian.stromme@qt.io>