CMYK JPEGs appearing inverted in PDF output

Addresses issue [#1128](#1128)

According to
[libjpeg](https://github.com/libjpeg-turbo/libjpeg-turbo/blob/3c17063ef1ab43f5877f19d670dc39497c5cd036/libjpeg.txt#L1569-L1582)
"it appears that Adobe Photoshop writes inverted data in CMYK JPEG
files"

An Adobe JPEG can be identified by the presence of the
[APP14](https://exiftool.org/TagNames/JPEG.html#Adobe)
segment.

The code now checks for the `APP14` segment in `RasterImage` and adds a
Decode Array to the XObject when rendering the CMYK JPEG. The value of
the Decode Array is the inverse of the default value for DeviceCMYK
according to the PDF spec. This has the effect of inverting the inverted
image back to normal.

weasyprint/images.py

@@ -66,6 +66,7 @@ def __init__(self, pillow_image, image_id, image_data, filename=None,
         self.height = pillow_image.height
         self.ratio = (self.width / self.height) if self.height != 0 else inf
         self.optimize = optimize = options['optimize_images']
+        self.app14 = getattr(original_pillow_image, 'app', {}).get('APP14')
 
         if pillow_image.format in ('JPEG', 'MPO'):
             self.format = 'JPEG'
@@ -150,6 +151,11 @@ def get_x_object(self, interpolate, dpi_ratio):
         })
 
         if self.format == 'JPEG':
+            if self.mode == 'CMYK' and self.app14 is not None:
+                # The presence of the APP14 segment indicates an Adobe image
+                # with inverted CMYK data. Specify a Decode Array to invert
+                # it again back to normal.
+                extra['Decode'] = '[1 0 1 0 1 0 1 0]'
             extra['Filter'] = '/DCTDecode'
             return pydyf.Stream([self.image_data], extra)