user-font with colour bitmap always comes out black
Poppler uses user fonts to render Type3 fonts embedded in a PDF. A Type3 font is a font comprised of PDF drawing operations. Those drawing operations can include drawing a bitmap, and commonly in the case of emoji, those bitmaps are colour. However, when using the poppler cairo backend these always come out as a black silhouette. As a result Evince renders PDFs with emoji embedded this way incorrectly.
See poppler/poppler#729 for the corresponding Poppler bug report.
I think this means that Cairo needs to support drawing colour bitmaps in user fonts.