Some SVG features aren't supported for SVG app icons

[deansheather]

Embedded bitmaps

Some SVG files are actually PNGs masquerading as SVGs.

Take for example /icon/cursor.svg in Coder:

<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="36" height="36" fill="none">
  <path fill="url(#a)" d="M0 0h36v36H0z"/>
  <defs>
    <pattern id="a" width="1" height="1" patternContentUnits="objectBoundingBox">
      <use xlink:href="#b" transform="scale(.0005)"/>
    </pattern>
    <image xlink:href="data:image/png;base64,iVBOR..." id="b" width="2000" height="2000"/>
  </defs>
</svg>

WinUI 3's SvgImageSource doesn't seem to be able to render <image> tags like these in SVGs, so cursor.svg shows up as blank.

There's no way to detect that it failed to render as it fails silently.

VS Code Icon gradient

The VS Code icon /icon/code.svg has a weird white gradient over it in the app that doesn't appear when viewing the SVG in a web browser.

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We could fix all of the SVGs in Coder and just declare that the desktop app for windows can't render icons like these somewhere in our documentation or something, or we could parse the SVG to look for these tags and try to render as a BitmapImage instead.

Relates to #91