Improve position:fixed support in layout_2020
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This makes it so that position:fixed elements do not scroll with the
rest of the contents, but does not tackle the rest of the issues with
their positioning.
Add CRLF to encoded multipart form data
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Some (3) WPT tests were failing because they expected the body for a multipart form data response to end with a CRLF. So I updated encode_multipart_form_data to add the missing terminator.
Looking at the corresponding spec (https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#multipart%2Fform-data-encoding-algorithm) and RFC (https://tools.ietf.org/html/rfc7578), I couldn't find anything mentioned about this detail.
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Some WPT tests were failing because they expected the body for a
multipart form data response to end with a CRLF. So I updated
encode_multipart_form_data to add the missing terminator.
Enable webxr by default
This changes two things:
* enable WebXR by default, since it's not exactly an experimental technology in Servo anymore
* enable the glwindow webxr backend by default so it's easier to test WebXR on desktop nightlies
Since the glwindow pref is only checked in the glutin desktop port, enabling it for other ports does not hurt us at all and it removes a papercut for testing WebXR content on desktop.
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Async wasm compilation event loop integration
The PR contains changes related to binding the runnable dispatching in script_runtime and is part of the Asynchronous WebAssembly Compilation fix. This is the first step in the subsequent steps mentioned in the [wiki](https://github.com/servo/servo/wiki/Asynchronous-WebAssembly-compilation-project).
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Fix loophole in messageport transfer
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layout: Ensure abs-pos elements establish block formatting contexts
Updating #16310
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Reduce unnecesarily large borrow in script thread
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- [x] These changes fix#24730
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