Support alpha masks in CMYK, Greyscale

[scoiatael]

Hey,

First of all, thanks for a great library - at OpenDesign we started using it to extract data from Adobe Illustrator files :) Hope to bring more contributions in the future.

About this PR - looks like alpha masks are used extensively in these kinds of files. so I added simple support for them. Unfortunately the code as it is converts everything into NRGBA - meaning we might lose some colour/format specific info - but I don't know these formats well enough.

Let me know if and how I should fix it and I'll be happy to do so :)

[CLAassistant]

All committers have signed the CLA.

[hhrutter]

Thanks for taking the time!
Please provide a small sample image for each new image type you provide support with this PR.
I'd like to add corresponding tests.

[scoiatael]

Sure - let me circle back in around a week, since the files I've been testing on are proprietary and I'll need to handcraft something in Illustrator :)

[scoiatael]

Added sample file and test for CMYK softmasks -> file is taken from here: https://365psd.com/vector/red-eaten-apple-by-ray-craighead-27023 - turned out to be easier to just find one than create it by hand.
Sadly can't find anything to test DeviceGray with.

[hhrutter]

Thanks! I'll check it out.

[hhrutter]

Hello,
Unfortunately the timing was not right for a merge in when you initially opened this PR - sorry..
Please get the latest release and ensure all tests pass, then I am happy to merge in your contribution.
Thank you!

[hhrutter]

This is merged into the latest release.