Optimise ImageOps.fit by combining resize and crop

[homm]

This optimises crop+resize operations sequence by using box parameter of resize function. This gives us:

1. Performance. Results for lilliput-bench:

Before:

JPEG 256x256 => 32x32:  1109 Bytes,     avg: 1.43 ms    min: 0.92 ms    max: 4.43 ms
PNG 256x256 => 32x32:   2603 Bytes,     avg: 3.70 ms    min: 3.62 ms    max: 7.14 ms
WEBP 256x256 => 32x32:  654 Bytes,      avg: 2.77 ms    min: 2.48 ms    max: 6.30 ms
GIF 256x256 => 32x32:   1897 Bytes,     avg: 4.89 ms    min: 4.51 ms    max: 11.58 ms
JPEG 1920x1080 => 800x600:      107480 Bytes,   avg: 26.28 ms   min: 24.90 ms   max: 37.08 ms
PNG 1920x1080 => 800x600:       810489 Bytes,   avg: 282.60 ms  min: 277.43 ms  max: 377.78 ms
WEBP 1920x1080 => 800x600:      84960 Bytes,    avg: 126.52 ms  min: 123.86 ms  max: 149.28 ms
GIF 1920x1080 => 800x600:       411470 Bytes,   avg: 102.40 ms  min: 95.52 ms   max: 126.27 ms

After:

JPEG 256x256 => 32x32:	1109 Bytes,	avg: 0.95 ms	min: 0.74 ms	max: 4.93 ms
PNG 256x256 => 32x32:	2603 Bytes,	avg: 3.56 ms	min: 3.45 ms	max: 7.60 ms
WEBP 256x256 => 32x32:	654 Bytes,	avg: 2.77 ms	min: 2.47 ms	max: 6.36 ms
GIF 256x256 => 32x32:	1897 Bytes,	avg: 4.73 ms	min: 4.37 ms	max: 11.39 ms
JPEG 1920x1080 => 800x600:	107450 Bytes,	avg: 21.36 ms	min: 20.99 ms	max: 27.02 ms
PNG 1920x1080 => 800x600:	810516 Bytes,	avg: 277.31 ms	min: 272.55 ms	max: 328.37 ms
WEBP 1920x1080 => 800x600:	84952 Bytes,	avg: 125.57 ms	min: 121.98 ms	max: 154.98 ms
GIF 1920x1080 => 800x600:	411470 Bytes,	avg: 98.35 ms	min: 94.72 ms	max: 117.11 ms

2. Accuracy. Unlike crop method, box for resize method accepts float coordinates. For example:

from PIL import Image, ImageOps
dot = Image.open('../dot.png')
ImageOps.fit(dot, (220, 200), Image.BICUBIC).save('../_out.png')

Source image (50x30px), processed image before, processed image after:

[hugovk]

Let's remember to add this to the release notes.

[homm]

@hugovk Added release notes. Could you check?

[hugovk]

Thanks!

[hugovk]

Suggested change

	Now uses the one resize operation with ``box`` argument internally
	Now uses one resize operation with ``box`` argument internally

[hugovk]

Suggested change

	instead of the crop and scale operations sequence.
	instead of a crop and scale sequence.

[hugovk]

Suggested change

	``box`` parameter accepts float values.
	the ``box`` parameter accepts float values.