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.

examples/boids_2d/main.cpp

@@ -91,7 +91,7 @@ class Boids2D : public sophia::Application {
         {{-BOID_LENGTH * 0.5f, BOID_WIDTH * 0.5f}, {1.0f, 1.0f, 1.0f}},
         {{-BOID_LENGTH * 0.5f, -BOID_WIDTH * 0.5f}, {1.0f, 1.0f, 1.0f}},
     };
-    boidMesh = assets().loadMesh(std::move(vertices), {});
+    boidMesh = resources().create<sophia::Mesh>(std::move(vertices));
   }
 
   void onUpdate(const sophia::FrameInfo& frameInfo) override {