Adds FlVsyncWaiter, wiring args.vsync_callback so the engine stops using
VsyncWaiterFallback's hardcoded 60Hz timer.

NOTE: on its own this is a regression - the engine produces frames faster
than GTK paints them, because the paint is still scheduled on GDK's frame
clock. Kept as groundwork for a presentation path that does not go through
gdk_cairo_draw_from_gl.

Co-Authored-By: Claude Opus 5 <[email protected]>

Follow-ups from adversarial review of the frame pacing work:

- Register the engine vsync callback only for the Vulkan backend. The
  OpenGL and software paths present through GTK's paint cycle, which GDK
  caps at 60Hz for GL windows (flutter#183703); pacing production
  above presentation only renders frames that are never shown (measured:
  93 produced vs 57 presented per second on the GL path).

- Add a timeout guard to the frame clock path of the vsync waiter. The
  clock does not tick while the window is unmapped, and the runner maps
  the window on the first frame, which deadlocked X11 at startup.

- Declare the subsurface size from realize. GTK allocates before it
  realizes, so the initial size_allocate ran before the Vulkan manager
  existed and the wp_viewport destination was never set: under a 1.5
  fractional scale the first frame rendered 1.5x too large, overflowing
  the window until the first manual resize.

- Stop committing the child surface from the platform thread in
  set_surface_size. The viewport destination is double-buffered state
  that Impeller's raster thread commits on every present; committing
  from the platform thread interleaved with the WSI's attach/commit.

- Announce the first frame from the embedder's SetNextFrameCallback
  (via a new FlRenderable.notify_first_frame) instead of an idle source
  that captured the widget unsafely and fired before any frame existed.

- Fix a per-call FlRenderable reference leak in the vsync waiter.

- Update RendererTypeVulkanEnvVar test to the new selection behaviour,
  drop dead APIs (set_subsurface_sync, get_extent), and rewrite the
  hand-written viewporter header in plain C.

Co-Authored-By: Claude Fable 5 <[email protected]>

Formatting hook fix only; no content change.

Co-Authored-By: Claude Fable 5 <[email protected]>