Remove fully opaque transparent channels

[BertrandBordage]

As discussed in #66, it would be great to detect when a transparent channel is fully opaque and remove it before saving the new image.

This speeds up encoding while decreasing file size by around 10%.

Implementing this feature would essentially be:

from PIL import Image

image = Image.open(original_filename)
if image.mode == 'RGBA':
    colors = image.getchannel('A').getcolors()
    if len(colors) == 1 and colors[0][1] == 255:
        image = image.convert('RGB')
image.save(destination)

[zerolab]

@BertrandBordage I think you meant this for wagtail/wagtail rather than wagtail/Willow ;)

[BertrandBordage]

@zerolab No I didn’t, see #66 ;)

[zerolab]

I take it back, misread "open one on Willow about removing empty alpha channels". Sorry for the noise!