🐛(frontend) keep commented text sharp when printing to PDF

Gecko cannot express a blended run in PDF, so it falls back to a bitmap of that
run: every commented sentence came out of Print as a 72 dpi image while the rest
of the page stayed text. The print stylesheet already hid the comment highlight,
but only its background and border, so mix-blend-mode stayed and that is what
forces the fallback.

Measured on the engine the report came from, printing the same page twice:
with the current rule the commented run is not extractable and the file carries
an image plus its alpha mask, with mix-blend-mode neutralised it is text again
and the file carries no image at all.

Signed-off-by: tanguy chenier <tanguychenier@gmail.com>

src/frontend/apps/impress/src/features/docs/doc-export/utils_print.ts

@@ -35,11 +35,12 @@ const PRINT_ONLY_CONTENT_CSS = `
     display: none !important;
   }
 
-  /* Hide selection highlights */
+  /* Hide selection highlights. */
   .ProseMirror-yjs-selection,
   .bn-thread-mark {
     background-color: transparent !important;
     border-bottom: none !important;
+    mix-blend-mode: normal !important;
   }
 
   /* Reset all layout containers for print flow */