theme: explicitly set the width/height of the Matrix icon

Chrome and Safari default <svg> width and height to 'auto', which
expands the viewport to fit the pixels within. Safari doesn't, and
the viewport collapses to 0x0 unless explicitly expanded.

Firefox calculates the viewport's natural dimensions to be 16x18,
so I used those values.

Fixes #8

themes/lix/layouts/partials/icons/matrix.html

@@ -1,4 +1,4 @@
-<svg version="1.1" viewBox="0 0 27.9 32" xmlns="http://www.w3.org/2000/svg" xmlns:cc="http://creativecommons.org/ns#" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
+<svg version="1.1" viewBox="0 0 27.9 32" width="16px" height="18px" xmlns="http://www.w3.org/2000/svg" xmlns:cc="http://creativecommons.org/ns#" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
  <title>Matrix (protocol) logo</title>
  <g transform="translate(-.095 .005)" fill="#040404">
   <path d="m27.1 31.2v-30.5h-2.19v-0.732h3.04v32h-3.04v-0.732z"/>