Fix Alpha channel not respected on graphics

[zenocross]

Closes #19978

For reference, the structure of the pdf file is the ff:

/BBox [
    167.119995
    261.677
    311.02899
    274.83899
  ]
  /Filter /FlateDecode
  /FormType 1
  /Group <<
    /S /Transparency
    /Type /Group
  >>
  /Length 85
  /Matrix [
    1
    0
    0
    1
    -167.119995
    -261.677
  ]
  /Resources <<
    /ExtGState <<
      /TransGs <<
        /AIS false
        /BM /Multiply
        /CA 1
        /Type /ExtGState
        /ca 1
      >>
    >>
    /ProcSet [
      /PDF
    ]
  >>
  /Subtype /Form
  /Type /XObject
`

[calixteman]

Interesting, but I don't think it's the right way to fix the bug unfortunately...
I slightly modified the pdf in order to add an other drawing in the group:
foo.pdf

While the rendering is the same in Chrome and Acrobat, it's wrong with pdf.js with or without your patch.
I don't know if there is a way to fix this bug, probably something like:
https://github.com/fserb/canvas2D/blob/master/spec/layers.md
could help.