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.

testbed/main.cpp

@@ -1,17 +1,35 @@
-#include <iostream>
+#include <imgui.h>
+
 #include <sophia/application.hpp>
+#include <sophia/frame_info.hpp>
+#include <sophia/ui/panel.hpp>
+
+class Viewport : public sophia::Panel {
+ public:
+  Viewport(sophia::UI& ui, sophia::RenderTargetHandle target)
+      : Panel{{.name = "Viewport"}}, ui{ui}, target{target} {}
+
+  void onUpdate(const sophia::FrameInfo&) override {
+    ImGui::Text("Test ViewPort");
+
+    ui.image(target);
+  }
+
+ private:
+  sophia::UI& ui;
+  sophia::RenderTargetHandle target;
+};
 
 class Testbed : public sophia::Application {
  public:
   Testbed() : Application{{.width = 750, .height = 1000, .name = "Testbed"}} {}
 
   void onCreate() override {
-    std::cout << "Testbed created" << std::endl;
-
-    debugUI().show();
+    sophia::RenderTargetHandle target =
+        resources().create<sophia::RenderTarget>();
+    setSceneTarget(target);
+    panels().emplace<Viewport>(ui(), target);
   }
-
-  void onUpdate(const sophia::FrameInfo& frameInfo) override {}
 };
 
 SOPHIA_MAIN(Testbed)