GLFW example with dirty region management

[betrevisan]

The changes proposed in this PR update the GLFW example to support dirty region management. It updates the example to a loading spinner in the center of the screen and adjusts the user-defined embedder callbacks to manage the information necessary for partial repaint. The goal of this example is also to serve as a local benchmark for dirty region management within the Embedder API.

Note: This PR is dependent on https://github.com/flutter/engine/pull/35022.

Pre-launch Checklist

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[bdero]

Given the additional complexity and build requirements not available on MacOS (EGL), can we make this a separate GLFW example? Perhaps under examples/glfw_drm with a README.md explaining what DRM is/pointing to the doc? It would be nice to keep example/glfw as the simplest practical demonstration for people just getting started with embedding Flutter.

[betrevisan]

Given the additional complexity and build requirements not available on MacOS (EGL), can we make this a separate GLFW example? Perhaps under examples/glfw_drm with a README.md explaining what DRM is/pointing to the doc? It would be nice to keep example/glfw as the simplest practical demonstration for people just getting started with embedding Flutter.

I agree that it would be nice to keep the example as simple as possible. I will add the partial repaint example as an extra then.

[bdero]

This looks like it should be good to go as soon as https://github.com/flutter/engine/pull/35022 lands