Investigate writing tiles out of order

[rleigh-codelibre]

The current behaviour when writing a plane is to flush tiles sequentially. This was, I think, done for the purpose of strict compatibility with the Bio-Formats TiffWriter/TiffSaver which strictly require writing in order. Looking at the libtiff documentation, libtiff does not have this restriction, and it should be possible to flush files as soon as they are completed. It would even be possible to support rewriting. This would require the IFD flush method to be adjusted to update the behaviour for coverage checking and tile ordering.

See this mail for further details.