Change goroutine implementation to use "for ... := range ...".
Merge branch 'goroutine-writer' of github.com:mawicks/PDFiG into goroutine-writer

Conflicts:
	pdf/file.go
Refactor to eliminate access to xref structure in goroutine because it wasn't safe.
Also clean up handling of pre-existing Indirect objects.
Merge branch 'goroutine-writer'