added GNU Free Font to be used by sales PDFs

[rossmc]

Please Note: A similar PR(#13016) I submitted for the same issue to the 2.2-develop branch has been merged already.

Description

The sales PDFs use the Libertine font which is not a unicode font and therefore it does not have full support of characters. It does not support Arabic, Russian, Greek, Indian or Thai alphabets and many more. For example it does not support the Indian rupee currency symbol ( ₹ ) and displays tofu boxes instead, which makes the PDF invoice totals illegible.

It's better if we use a unicode font to support more writing systems and characters. So I've added the open source GNU Free Font which supports much more writing systems.

Fixed Issues (if relevant)

1. Magento 2.1.6 - Invoice PDF doesn't support Thai #9666: Magento 2.1.6 - Invoice PDF doesn't support Thai
2. Magento 2.1.3 - Invoice and shipment PDF doesn't support Arabic #12323: Magento 2.1.3 - Invoice and shipment PDF doesn't support Arabic

Manual testing scenarios

1. Set the store currency to use the ruppe currency symbol ( ₹ ) .
2. Print an invoice.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @rossmc. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.1.15 release.