Ignore `pageColors` when the background/foreground is identical (PR 14874 follow-up)
Ignore `pageColors` when the background/foreground is identical (PR 14874 follow-up)

If the computed background/foreground colors are identical, the `canvas` would be rendered mostly blank with only images visible. Hence it seems reasonable to also ignore the `pageColors`-option in this case.

Also, the patch tries to *briefly* outline the various cases in which we ignore the `pageColors`-option in a comment.

Shoobx/pdf.js,showpad/mozilla-pdf.js,mozilla/pdf.js,showpad/mozilla-pdf.js,Snuffleupagus/pdf.js,nawawi/pdf.js,timvandermeij/pdf.js,nawawi/pdf.js,nawawi/pdf.js,Snuffleupagus/pdf.js,Snuffleupagus/pdf.js,showpad/mozilla-pdf.js,timvandermeij/pdf.js,mozilla/pdf.js,Shoobx/pdf.js,timvandermeij/pdf.js,mozilla/pdf.js,Shoobx/pdf.js