Basic recording support for loaded captures
Fixes#2583 (more follow-up is expected though for correctness)
This change makes the capture infrastructure play better with the display-list-based recording. It produces a fake `SetDisplayList` message to be recorded based on the (already) loaded scene. Result isn't always consumable by the YAML reader (there are correctness issues, and I bet the YAML reading-writing loop isn't sinless either, given that it was never tested on big scenes, typically produced by capturing from Gecko). Here is the `basic` example replayed from YAML:
YAML is at least always generated, and it gives an easier workflow for developers trying to reproduce a captured bug in a YAML reftest.
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