Fix #4: Change masked layer size to same as other channels

[mdboom]

The masked channel in a layer does not have to be the same size as the other channels. That gets pretty inconvenient to work with, so when using the "user-friendly" interface in user.nested_layers, the masked channel will now get forcibly cropped/extended to the same size as the other channels.

With this change, it should be possible to make a PNG that applies the masked layer as follows:

import pytoshop
from pytoshop.user import nested_layers
import numpy as np
from PIL import Image

psdfile = pytoshop.read(open("masked_layer.psd", 'rb'))

layers = nested_layers.psd_to_nested_layers(psdfile)

l = layers[0]

a = np.array(l.channels[-1].image * (l.channels[-2].image / 255), np.uint8)
rgba = np.dstack(
    [l.channels[0].image, l.channels[1].image, l.channels[2].image, a]
)

im = Image.fromarray(rgba)
im.save('test.png')

[codecov-io]

Codecov Report

Merging #5 into master will increase coverage by 0.15%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage   89.66%   89.82%   +0.15%     
==========================================
  Files          14       14              
  Lines        3067     3085      +18     
==========================================
+ Hits         2750     2771      +21     
+ Misses        317      314       -3

Impacted Files	Coverage Δ
pytoshop/user/nested_layers.py	90.36% <100%> (+1.11%)	⬆️
pytoshop/layers.py	89.88% <0%> (+0.14%)	⬆️

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