Liberation Sans font source and distribution notices

The four ../standard_fonts/LiberationSans-*.ttf assets are unmodified files
from pdfjs-dist 6.4.299. Each is byte-identical to Mozilla's v6.4.299
external/standard_fonts asset. Their font metadata identifies version 1.07.4.

Font license: GNU General Public License version 2 with the Liberation font
exception. See ../standard_fonts/LICENSE_LIBERATION, COPYING and License.txt.
The document embedding exception is part of that notice. No font was renamed,
rebuilt, or converted by CoreLinks.

liberation-fonts-1.07.4.tar.gz is the complete unmodified official preferred
SFD source release, including source files, Makefile, export scripts and
copyright/license notices. It is provided for recipients to inspect and
modify the fonts. It was acquired from:
https://releases.pagure.org/liberation-fonts/liberation-fonts-1.07.4.tar.gz

Provenance:
https://raw.githubusercontent.com/mozilla/pdf.js/v6.4.299/external/standard_fonts/README.md
https://github.com/mozilla/pdf.js/pull/21750
https://github.com/liberationfonts/liberation-1.7-fonts

Correspondence limitation: the official 1.07.4 binary archive is not
byte-identical to Mozilla's font assets. Mozilla's files have 681 glyphs;
the official binary has 682 Regular/Bold and 687 Italic/BoldItalic glyphs.
All 668 Unicode mappings, glyph IDs and advance widths match for each style.
Raw table/glyph bytes differ. Source rebuilding and pixel/outline equality
have not been established. See font-table-comparison.json and
glyph-correspondence.json; see ../../../docs/pdf-font-acquisition-0.7.json
in the review source package for acquisition URLs, hashes and full evidence.

The additional fonts preserve PDF.js's expected glyph ordering. Replacing
them with Liberation 2.x merely by renaming files is not a supported change:
PDF.js's glyph width/scale tables must be regenerated when font files change.
