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This PR adds multiview support for `ExternalViewEmbedder`.
## Nomenclature
The term **view** can be ambiguous in `ExternalViewEmbedder`, and therefore the following terms are used:
* A **native view** refers to the final drawing surface that to composite layers to. It is the "view" used in other places of the engine, such as `Shell::AddView`.
* A **platform view** refers a platform view, a layer that holds content to be embedded.
## Change
The lifecycle of `ExternalViewEmbedder` is changed:
<table>
<thead>
<tr>
<th>Before PR</th>
<th>After PR</th>
<th>How it's called</th>
</tr>
</thead>
<tbody>
<tr>
<td rowspan=2>BeginFrame</td>
<td>BeginFrame</td>
<td>Once per frame</td>
</tr>
<tr>
<td>PrepareFlutterView</td>
<td>Once per flutter view</td>
</tr>
<tr>
<td>SubmitFrame</td>
<td>SubmitFlutterView (renamed)</td>
<td>Once per flutter view</td>
</tr>
<tr>
<td>EndFrame</td>
<td>EndFrame (unchanged)</td>
<td>Once per frame</td>
</tr>
</tbody>
</table>
* `BeginFrame` should perform per-frame actions, such as merge-unmerging threads.
* `PrepareView` should perform per-native-view preparations, such as recording the view ID and view size.
This change is necessary because some actions in `PrepareView` needs to be refreshed at the beginning of drawing every native view.
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
Flow
Flow is a simple compositor based on Skia that the Flutter engine uses to cache recorded paint commands and pixels generated from those recordings. Flow runs on the raster thread and uploads information to Skia.