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.

editor/panels/viewport_panel.cpp

@@ -24,8 +24,9 @@ void ViewportPanel::onUpdate(const sophia::FrameInfo&) {
   const ImVec2 size = ImGui::GetContentRegionAvail();
   const ImVec2 end(origin.x + size.x, origin.y + size.y);
 
+  ui.image(target);
+
   ImDrawList* drawList = ImGui::GetWindowDrawList();
-  drawList->AddRectFilled(origin, end, IM_COL32(24, 26, 32, 255));
 
   if (showGrid) {
     constexpr float GRID_STEP = 40.0f;