Parallelize decoding

[chrstphrbrns]

Parallelizing decoding is straight-forward with TIFFs since they are composed of separately-encoded strips or tiles

One might say this should be done at the client/file level, but for large files it may be convenient to have it done at the strip level, especially in an interactive context

Here are results I get for a variety of LZW-compressed images--from tiny to huge, for white noise and solid color--showing a win for all but the smallest, simplest images

white noise

serial (s)	parallel (s)
25.06848	3.21034
2.06287	0.30021
0.27048	0.07222
0.06873	0.02438
0.00174	0.00331

solid color

serial (s)	parallel (s)
1.35969	0.88756
0.08941	0.05104
0.00777	0.01789
0.00193	0.00331
0.00012	0.00023

[tlnagy]

💯 This would be great to have, it would potentially resolve #19 as well. I agree that the TIFF format lends itself nicely to parallelization. I just haven't had the bandwidth to tackle it, but would review any PR if you want to tackle it (looks like you might have a first draft already?)

The small TIFF issue could be fixed with some simple logic to parallelize if an image above a certain size/# of strips/etc