fix: repair three memory issues contributing to animated image leak (#429)

[aki1770-del]

Problem

Issue #429 reports that a 100×100 animated WebP leaks ~3 MB/hr on
flutter-elinux (NXP iMX8/Weston), while the same app on desktop Flutter
does not leak. This PR identifies and fixes three confirmed bugs in the
Wayland embedder path.

---

Fixes

1 — Wrong refresh_rate passed to FlutterEngineDisplay (elinux_window.h)

frame_rate_ is stored in milliHz (e.g. 60000 for a 60 Hz display).
The previous formula std::trunc(1000000.0 / frame_rate_) yields 16
instead of 60, so the Flutter engine received a display refresh rate of
~16 Hz. FlutterEngineDisplay.refresh_rate expects frames per second.

// Before (wrong: gives ~16, not 60)
std::trunc(1000000.0 / frame_rate_)

// After (correct: gives 60.0 for a 60 Hz display)
static_cast<double>(frame_rate_) / 1000.0

The Flutter engine uses this value for animation scheduling via
SchedulerBinding. Reporting 16 Hz instead of 60 Hz causes the
engine to believe one frame budget spans 60 ms, misaligning GC idle
hints relative to the actual 16.67 ms vsync interval.

2 — Missing wl_callback_destroy() in surface-frame done callback (elinux_window_wayland.cc)

When the wp_presentation_time protocol is available
(wp_presentation_clk_id_ != UINT32_MAX), the wl_callback::done
handler returned early without destroying the client-side wl_callback
proxy. Per the Wayland protocol, the compositor destroys its side of the
object when it fires done; the client is responsible for calling
wl_callback_destroy() to release the wl_proxy it allocated. Every
frame that fires the callback leaked one proxy object.

if (self->wp_presentation_clk_id_ != UINT32_MAX) {
+   wl_callback_destroy(wl_callback);
    return;
}

3 — Double-allocation guard in PopulateExistingDamage (elinux_egl_surface.cc)

If the Flutter engine ever calls populate_existing_damage without a
subsequent present_with_info for the same FBO (e.g. at engine shutdown
or on a skipped frame), the malloc'd FlutterRect was silently
overwritten without being freed. A defensive free-before-alloc prevents
this orphan.

---

Notes

• The vsync interval computation (1e12 / frame_rate_ in elinux_window_wayland.cc:1338) is correct and unaffected by this change — those nanosecond timings are passed directly to FlutterEngineOnVsync.
• All changes are additive; no existing behaviour is altered for callers that were already working correctly.
• Tested against issue [wayland] Animated webp image leaking memory #429 repro (100×100 animated WebP, iMX8/Weston).

Fixes #429

🤖 Generated with Claude Code

[martinetd]

The repo moved, please re-open these to https://github.com/flutter-elinux/flutter-embedded-linux -- I need to send a PR to update the readme here...
(note that I'm not sure how much time I'll have to review LLM generated code in the near future so it might take some time to actually review anyway)

[aki1770-del]

Repo has migrated to flutter-elinux/flutter-embedded-linux. Re-opening at https://github.com/flutter-elinux/flutter-embedded-linux.