Added saving RGBA images as PDFs #6925
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hugovk
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radarhere
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Resolves #6924
As suggested in the issue, adds RGBA saving for PDFs through the JPXDecode filter with JPEG2000 image data.
However, when opening the result, I found that the background wasn't transparent, but black instead.
Testing converting a transparent PNG to JPEG2000 (
Image.open("Tests/images/transparent.png").save("transparent.jp2")), this is the case when simply saving JPEG2000 images. After looking at uclouvain/openjpeg#416 and https://github.com/uclouvain/openjpeg/blob/2d606701e8b7aa83f657d113c3367508e99bd12b/src/lib/openjp2/jp2.c#L2015-L2024 though, it turns out it was simply a case of telling OpenJPEG which channel was the alpha channel.