Restructure Blob, structured serialization
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FIX#24052 and also address the "cloning" half of FIX#23917
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Various webxr WPT fixes
Needs https://github.com/servo/webxr/pull/108
r? @asajeffrey
Went through most of the failing tests and fixed them. Many of the remaining ones fail due to unsupported features that I can slowly whittle away.
Improve diagnostics for WPT failures
* Include the full output (including stdout/stderr) in the intermittent-filtered log
* Print the intermittent-filtered log at the end of the main log (which is one less click to reach from Taskcluster’s task view, compared to other task artifacts)
* <del>Fail with a specific message when a reftest screenshot is entirely white</del> (This caused over a hundred unexpected results. A few of them in reftests that use `about:blank` as a reference.)
* For failing reftests, add a message if the whole screenshot is a solid color, to help recognize instances of https://github.com/servo/servo/issues/24726
```
▶ FAIL [expected PASS] /css/CSS2/box-display/root-box-003.xht
│ → /css/CSS2/box-display/root-box-003.xht 54a9df64f1476dd12020019d7cf22ac34d727bc0
│ → /css/CSS2/box-display/root-box-003-ref.xht 636eb693bc214b6e1c64e6566c48e69e6777b946
└ → Screenshot is solid color 0xFFFFFF for /css/CSS2/box-display/root-box-003.xht
```
(The last line is new.)
Issue 21810/improve validation methods
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This is a start at addressing #21810. I'm putting these changes out early to get some feedback on the following items:
1. I added unit tests for the validation methods mentioned in #21810, because I couldn't tell whether any of the existing WPT tests covered them. Are these tests worthwhile? Are any of them unnecessary?
2. I changed the implementation for `is_valid_floating_point_number_string` so that it passed the tests. The previous version of the function wasn't restrictive enough (it allowed certain whitespace characters before the number string).
3. I changed the catch-all condition in `htmlinputelement.rs` to account for the remaining input types that don't have a value sanitization algorithm. This last change seems good to me since we won't be able to add a new input type without adding it to the case and checking the spec for an algorithm.
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- [x] These changes fix#21810
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