Integrate with Unicode’s text rendering tests

[brawer]

Would you be interested in testing font.rs through Unicode text rendering tests?

I’d volunteer to do the integration into the test suite on the Unicode side, if somebody familiar with Rust writes a command-line tool that can be called like this:

path/to/some-command --font=fonts/TestCMAP14.otf --testcase=Foo-5/6 --render=≩≩

writing an SVG file to stdout like this:

<?xml version="1.0" encoding="UTF-8"?>
<svg version="1.1"
    xmlns="http://www.w3.org/2000/svg"
    xmlns:xlink="http://www.w3.org/1999/xlink"
    viewBox="0 -120 1446 1200">
  <symbol id="Foo-5/6.uni2269" overflow="visible"><path d="M100,334 L623,563 L623,619 L100,880 L100,793 L518,594 L100,420 Z M100,208 L622,208 L622,287 L100,287 Z M100,38 L622,38 L622,117 L100,117 Z M282,-93 L508,379 L436,413 L211,-59 Z"/></symbol>
  <use xlink:href="#Foo-5/6.uni2269" x="0" y="0"/>
  <use xlink:href="#Foo-5/6.uni2269" x="723" y="0"/>
</svg>

For test cases on variable fonts, your tool would be called with an additional argument for the variation settings, such as --variation=wght:481.3;wdth:70. Initially, you could ignore this.

The test fonts are here. If you write a command-line tool that can be called as described, the generated test report will look like this, this, this, or this.

[cbreeden]

cc me.

[cbreeden]

Referencing https://github.com/unicode-org/text-rendering-tests:

With --engine=FreeStack, the tests are run on the free/libre open-source text rendering stack with FreeType, HarfBuzz, FriBidi, and Raqm. These libraries are used by Linux, Android, ChromeOS, and many other systems. — Test report for FreeStack.

I think font-rs would be best to focus only on the FreeType part of this stack. To handle this, we would need to add the proper FriBidi and HarfBuzz plumbing to get these tests to work.