Fix Linux handling of window exposure events

[OmarEmaraDev]

Description

Currently, the Linux embedder does not handle window exposure events.
This is typically not a problem for users who use compositing window
managers, since they keep the display buffers even if the window is
completely covered. However, for users that don't use a compositor, the
window will not be redrawn by the engine if it was previously covered
until another event triggers the redraw.

This patch implements the GtkWidget draw callback to handle window
exposure events. The callback doesn't actually draw anything, it just
schedule a frame for drawing by the engine.

The engine doesn't support exposure events, so instead, we force redraw
by sending a window metrics event of the same geometry. Since the
geometry didn't change, only a frame will be scheduled.

Related Issues

Fixes flutter/flutter#68391

Tests

I haven't added any tests. Any pointers as how I should add tests for this?

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the contributor guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the C++, Objective-C, Java style guides for the engine.
• I read the tree hygiene wiki page, which explains my responsibilities.
• I updated/added relevant documentation.
• All existing and new tests are passing.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• No, no existing tests failed, so this is not a breaking change.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat.

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[robert-ancell]

I was concerned there might be side effects (e.g. double rendering of a frame) but this will be resolved by future API in flutter/flutter#69716. This change seems like a suitable fix for now, thanks @OmarEmaraDev! LGTM.