Do not present the root surface when an external view embedder owns it - #193000

GPUSurfaceVulkan and GPUSurfaceGLImpeller both take render_to_surface and
hand back a no-op SurfaceFrame when it is false. GPUSurfaceVulkanImpeller
has no such flag, so with a compositor the engine acquires an embedder
image and presents it after every composited frame.

The compositor has already presented that frame's layers. The root present
puts an unrendered image on top of them. An embedder holding one image sees
nothing; one cycling a ring of scanout buffers sees its frame replaced by
whatever the ring handed back -- one good frame, then black, at a flip rate
that still reads 60fps.

Reachable once the embedder can build an Impeller render target from a
Vulkan backing store (flutter#187525); the gate is the groundwork
for it.

Dispose of the thread-local pools before the gate rather than after. It is
per-frame upkeep, not part of presenting, and skipping it on the compositor
path would leave the raster thread holding its command and descriptor pools
for the life of the engine.

impeller_context_ is a shared_ptr<impeller::Context> handed in by the caller, and
it is often a SurfaceContextVK, which derives from Context rather than ContextVK.
ContextVK::Cast on it is an unchecked downcast: DisposeThreadLocalCachedResources
then locks desc_pool_mutex_ at the wrong offset, pthread_mutex_lock returns
EINVAL, and the process aborts. The same member is cast to SurfaceContextVK a few
lines below, so the two casts could not both be right.

DisposeThreadLocalCachedResources, CreateCommandBuffer and GetCommandQueue are
all virtual on Context and SurfaceContextVK overrides or delegates them, so call
through the base and drop the casts.

587a346 also changed the submit callback. That was unnecessary: the callback is
built inside the `delegate_ != nullptr` branch, the embedder backing-store path,
where impeller_context_ is a ContextVK -- the same branch as the GetDevice cast
below it. ContextVK::Cast is sound there.

Only DisposeThreadLocalCachedResources runs ahead of that branch, on both paths,
so it is the one place the context may be a SurfaceContextVK and the cast
undefined. That is the line andywolff flagged, and it stays fixed.

Restores the callback to ContextVK::Cast so the change is the single line.