AJI 86b582a6bc Fix damage calculation when not providing populate_existing_damage for gl embedder (flutter/engine#45611)
# Description

This PR fixes the `gl_populate_existing_damage` in embedder.cc, which currently returns an empty rectangle when a full repaint is needed. This leads to the `frame_damage` being considered empty in rasterizer.cc, causing incorrect partial repaint within Flutter when `gl_populate_existing_damage` is not provided by the embedder.

# Related Issue

https://github.com/flutter/flutter/issues/119601

# Tests

Add a new test. Also fixes damage calculation related tests in EmbedderTest by

* Use a new Dart embedder fixture entry point `render_gradient_retained` which retains the old layer to make sure layer tree diff happens.
* Add `latch.Wait()` after `SetGLPresentCallback` to make sure assertions have been executed.
* Make `existing_damage_rects` static since it should be valid after `populate_existing_damage` returns.
2023-09-28 20:06:51 +00:00
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