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.

List which issues are fixed by this PR. You must list at least one issue.

Add tests

Pre-launch Checklist

• [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• [x] I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• [ ] I listed at least one issue that this PR fixes in the description above.
• [ ] I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on writing and running engine tests.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I signed the [CLA].
• [ ] All existing and new tests are passing.