tiff: Allow flushing of tiles in any order

[rleigh-codelibre]

Closes: #97

Instead of flushing the tiles strictly in index order, flush them as soon as they are covered. This reduces the size of the tile cache, and spreads out disc I/O, if the tiles are written in a different order.

There is no change in usage. The API continues to allow saveBytes to be called in any order and with any size of region, which may or may not overlap with the tile size. The only change is when tiles are flushed from the tile cache, which is now earlier than before for certain usage patterns. All other aspects are unchanged.

Testing: Writing in different orders, including random order, with and without saveBytes regions overlapping tile boundaries, is already handled by the unit tests (tiff), including validation of the pixel data. Check that all builds remain green.

[rleigh-codelibre]

By the way, use a whitespace diff for a more readable diff.

[rleigh-codelibre]

@joshmoore @sbesson Any chance of getting this one merged? It's going to be a prerequisite for the pyramid writing PR which I would like to open in the next few days.

Thanks,
Roger

[rleigh-codelibre]

Closing because it will be incorporated into the subresolution writer changes.