Currently the embedding API only provides the embedder with the URL for
a favicon. This is not great, for multiple reasons:
* Loading the icon should happen according to the fetch spec which is
not easy for the embedder to recreate (consider CSP, timing information
etc)
* Rasterizing a svg favicon is not trivial
With this change, servo fetches and rasterizes the icon to a bitmap
which is then passed to the embedder.
Testing: I'm not sure how I can write tests for the embedding api. I've
tested the correctness manually using
https://github.com/servo/servo/pull/36680.
Prepares for https://github.com/servo/servo/pull/36680
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Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>