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;

editor/panels/viewport_panel.hpp

@@ -1,15 +1,25 @@
 #pragma once
 
 #include <sophia/frame_info.hpp>
+#include <sophia/handles.hpp>
 #include <sophia/ui/panel.hpp>
+#include <sophia/ui/ui.hpp>
 
 class ViewportPanel : public sophia::Panel {
  public:
-  ViewportPanel() : Panel{{.name = "Viewport"}} {}
+  ViewportPanel(sophia::UI& ui, sophia::RenderTargetHandle target)
+      : Panel{{.name = "Viewport",
+               .windowFlags = ImGuiWindowFlags_NoScrollbar |
+                              ImGuiWindowFlags_NoScrollWithMouse}},
+        ui{ui},
+        target{target} {}
 
   void onUpdate(const sophia::FrameInfo& frameInfo) override;
 
  private:
+  sophia::UI& ui;
+  sophia::RenderTargetHandle target;
+
   enum class GizmoMode { Move, Rotate, Scale };
 
   GizmoMode gizmoMode = GizmoMode::Move;