[Impeller] Document threading configuration with Vulkan. (flutter#44874)

Moving bits and pieces of the presentation this morning into docs so we can keep them up to date.

impeller/docs/vulkan_threading.md

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+# Threading in Vulkan
+
+The Vulkan backend uses a dedicated concurrent worker pool that is created along
+with the creation of the Vulkan context.
+
+Unlike other pools such as the IO worker pool, long running tasks may **not** be
+posted to this pool. This is because frame workloads can and often are
+distributed to workers in this pool. Having a potentially long running task
+(such as texture decompression) may potentially block frame critical tasks. The
+limitation of a separate pool for frame critical tasks is working around the
+separate limitation of not being able to specify a QoS to specific tasks and may
+be lifted in the future.
+
+There is also a separate component called the fence waiter which operates on its
+own thread. The purpose of the fence waiter is to ensure that the resource
+reference count lives at least as long as the GPU command buffer(s) that access
+this resource.
+
+Resource collection and pooling also happens on another thread called the
+resource manager. This is because touching the allocators is a potentially
+expensive operation and having the collection be done in a frame workload, or on
+the fence waiter thread may cause jank.
+
+With this overview, the total number of thread used by the Impeller Vulkan
+backend is the number of workers in the concurrent worker pool, and the two
+threads for the fence waiter and resource manager respectively.
+
+A summary of the interaction between the various threads is drawn below:
+
+
+```mermaid
+sequenceDiagram
+    participant rt as Render Thread
+    participant worker1 as Concurrent Worker 1
+    participant worker2 as Concurrent Worker 2
+    participant fence_waiter as Fence Waiter
+    participant resource_manager as Resource Manager
+    participant gpu as GPU
+    rt->>+worker1: Setup PSO 1
+    rt->>+worker2: Setup PSO n
+    worker1-->>-rt: Done
+    worker2-->>-rt: Done
+    Note over rt,resource_manager: Application launch
+    loop One Frame
+        activate rt
+        rt->>+worker2: Frame Workload
+        activate fence_waiter
+        rt->>fence_waiter: Resource 1 owned by GPU
+        worker2-->>-rt: Done
+        rt->>fence_waiter: Resource 2 owned by GPU
+        rt->>gpu: Submit GPU Commands
+        deactivate rt
+    end
+    activate gpu
+    gpu-->>fence_waiter: GPU Work Done
+    fence_waiter->>resource_manager: Collect/Pool Resources
+    deactivate fence_waiter
+    activate resource_manager
+    deactivate gpu
+    deactivate resource_manager
+```