Ignore the mesh target count before version 7

The ninth field of the Mesh struct only became a morph target count in
version 7. Before that it is a leftover subset offset, one of the fields
that have to be read and discarded, and the reader stored it in the
target buffer's numTargets regardless, so a version 3 file came back
claiming a couple of hundred thousand morph targets.

Nothing reads it in that state, since the renderer gates the morph path
on the target data being non-empty, but it is a garbage value that
disagrees with the format. Of the 114 .mesh files here 39 were
affected, and with this they agree field for field with an independent
implementation on 111 of the 114.

Change-Id: Ib9d6161adefadd45d48c6cae1d517162f693c596
Reviewed-by: Christian Strømme <christian.stromme@qt.io>