Add offscreen render targets and rename AssetRegistry to ResourceRegistry

Add a RenderTarget resource so the scene can render offscreen instead of
straight to the swapchain. The editor uses it to show the scene inside the
Viewport panel.

- Rename AssetRegistry to ResourceRegistry (AssetTraits -> ResourceTraits,
  assets() -> resources()) and move the pools from engine/src/assets to
  engine/src/resources. It now covers non-asset GPU resources as well.
- Replace loadMesh() with create<T>(...), and add create<RenderTarget>(),
  requestResize(), getExtent() and isAlive().
- Add RenderTarget and RenderTargetPool with RenderTargetHandle.
  Resizes are deferred and applied at the start of the next frame, when the
  GPU is idle.
- Add Application::setSceneTarget(). With no target set, the scene still
  renders to the swapchain. With a target set, the scene renders offscreen
  and the swapchain receives only the UI. Frame extent and camera projection
  follow the target's size.
- Add sophia::UI (Application::ui()) with image() and getTextureId() to draw
  a render target in an ImGui window, sizing the target to the available
  space. ImGuiRenderer now registers the target textures for this.
- ViewportPanel takes the UI and a target, and draws the scene image in place
  of the flat background fill.
- Update the testbed, the boids example, and the glossary for the rename.

engine/src/resources/pools/mesh_pool.hpp

@@ -0,0 +1,35 @@
+#pragma once
+
+#include <memory>
+#include <vector>
+
+#include "device.hpp"
+#include "mesh.hpp"
+
+namespace sophia {
+
+class MeshPool {
+ public:
+  explicit MeshPool(Device& device);
+  ~MeshPool();
+
+  MeshPool(const MeshPool&) = delete;
+  MeshPool& operator=(const MeshPool&) = delete;
+
+  MeshHandle create(const Mesh::Builder& builder);
+  const Mesh& get(MeshHandle handle) const;
+  void release(MeshHandle handle);
+
+ private:
+  struct Slot {
+    std::unique_ptr<Mesh> mesh;
+    u32 generation = 0;
+  };
+
+  Device& device;
+
+  std::vector<Slot> slots;
+  std::vector<u32> freeList;
+};
+
+}  // namespace sophia