Inform browsers on how to optimize text rendering

The `text-rendering` svg property provides a hint to the user agent
about how to optimize its text rendering.

In this particular case, the value specified for the `text-rendering`
property is `geometricPrecision`, indicating that the user agent should
emphasize geometric precision over legibility and rendering speed.

This is done because certain aspects of fonts — such as kerning — don't
scale linearly:

    In SVG, when text is scaled up or down, browsers calculate the final
    size of the text (which is the specified font size and the applied
    scale) and request a font of that computed size from the platform's
    font system. However, if a user requests a font size of, say, 9 with
    a scale of 140%, the resulting font size of 12.6 doesn't explicitly
    exist in the font system, so the browser rounds the font size to 12
    instead. This results in stair-step scaling of text.

If `geometricPrecision` is fully supported by the rendering engine, it
will allow users to scale text fluidly, making the text look good.

References:

  * http://www.w3.org/TR/SVG/painting.html#TextRenderingProperty
  * https://developer.mozilla.org/en-US/docs/Web/CSS/text-rendering

See also:

  * espadrine/gh-badges#5