Add transpose and speedup rotation

[homm]

This adds new Imaging.TRANSPOSE method for Image.transpose() operation. Also this speeds up Image.ROTATE_90 and Image.ROTATE_270.

Method	Before	After
Large 5120x2880 image
Flip left right	0.02635	0.0262
Flip top bottom	0.01105	0.011
Rotate 90	0.3706	0.04535
Rotate 180	0.02968	0.02933
Rotate 270	0.3786	0.04542
Transpose		0.04404
Small 586x387 image
Flip left right	0.0003951	0.0003891
Flip top bottom	0.0001121	0.000108
Rotate 90	0.000602	0.000457
Rotate 180	0.000464	0.000463
Rotate 270	0.0005708	0.0004828
Transpose		0.0004091

As you can see, this doesn't improve performance for relatively small images, because they whole fit in cache, but for large images this gives 8x speedup.

Tests pending.

[wiredfool]

Rewording to native english. I think this is what you mean.

For large images rotation is an inefficient operation in terms of CPU cache.
One row in the source image affects each column in destination.
Rotating in chunks that fit in the cache can speed up rotation 8x on a modern CPU.
A chunk size of 128 requires only 65k of CPU cache and is large enough
that the overhead from the extra loops are not apparent.

[wiredfool]

Looks good to me.

• The one API change is documented in the docstring.
• I've noted one comment anglicization.
• I want to check to see how well we're testing this code prior to merging, and possibly add tests to make sure that we're matching the original.

Question: Merge order of this vs the other ones? After #977 ?

[homm]

Have added tests. Merge order doesn't matter.

[homm]

You really need change standard test image size :) #947

[hugovk]

Yes, it would be good, but it took quite a while to regenerate all the test images. (If we do decide to re-generate them, I tried to mention how they were generated in the commit messages.) We should at aim for any new non-hopper test images committed to meet the #947 requirements.

You could always open a different, non-square test image, or do a resize/crop before continuing.

[homm]

Update tests with not square image and fix bugs in tests related to this!

[hugovk]

Great, non-square test images are definitely the way forward!

Do you think you could add some error tests to cover these lines:
https://coveralls.io/files/345689512#L150 - 153

And the other similar ones in the same file if you like.

[homm]

I think this is impossible, because those functions are used only from '_rotate', which creates temporary images with right mode and size.

[hugovk]

Sure, no problem. I'll let wiredfool have a look over it before merging.

Thanks for all these improvements!