Replace background rect with a path to save 6 bytes

[kuubeu]

Since rx="15%" is no longer used for backgrounds, maybe we should stop using rects altogether? It would save 6 bytes on each icon which uses background rects.

Here's how they compare side by side:

<rect fill="tan" width="512" height="512"/>
<path fill="tan" d="M0 0H512V512H0"/>

[edent]

Sounds good. Try it and see!

[kuubeu]

Closed by #758