Add support for im.mode = "LA" in PdfImagePlugin.py

[mnmtz]

Please add support for images with mode "LA" in the PdfImagePlugin:

elif im.mode == "LA":
    filter = "JPXDecode"
    colorspace = PdfParser.PdfName("DeviceGray")
    procset = "ImageB"  # grayscale

[radarhere]

Could you provide some background for why you chose JPXDecode, DeviceGray and ImageB?

[mnmtz]

I want to replace a png image with mode "LA" with a compressed jp2k image in a PDF file while keeping the alpha channel to reduce the size of the PDF. The provided code was a temporary fix to get this done.

[radarhere]

Ok, see what you think of #7299

[radarhere]

#7299 has now been merged