feat(动画): 添加角色动作合成纹理功能

实现角色动作的离屏渲染合成功能，支持获取合成纹理及其相关信息：
1. 新增CanvasActor用于离屏渲染
2. 新增RenderTexture封装FBO和纹理
3. 扩展Renderer支持离屏渲染到纹理
4. 为CharacterAnimation添加合成纹理生成逻辑
5. 在调试界面添加合成纹理预览功能

Frostbite2D/include/frostbite2D/graphics/renderer.h

@@ -9,9 +9,12 @@
 #include <frostbite2D/graphics/texture.h>
 #include <frostbite2D/graphics/types.h>
 #include <string>
+#include <vector>
 
 namespace frostbite2D {
 
+class RenderTexture;
+
 class Renderer {
 public:
   static Renderer& get();
@@ -22,6 +25,10 @@ public:
   void beginFrame();
   void endFrame();
   void flush();
+  bool beginRenderToTexture(RenderTexture& target, Camera* camera,
+                            const Color& clearColor = Color(0.0f, 0.0f, 0.0f, 0.0f),
+                            bool clear = true);
+  bool endRenderToTexture();
 
   void setViewport(int x, int y, int width, int height);
   void setWindowSize(int width, int height, float contentScaleX = 1.0f,
@@ -61,6 +68,9 @@ public:
   const RenderResolutionState& getResolutionState() const {
     return resolutionState_;
   }
+  bool isInitialized() const { return initialized_; }
+  bool isFrameActive() const { return frameActive_; }
+  bool isRenderingToTexture() const { return !renderTargetStack_.empty(); }
   Vec2 screenToVirtual(const Vec2& screenPos) const;
   Vec2 virtualToScreen(const Vec2& virtualPos) const;
 
@@ -107,6 +117,17 @@ private:
   void recalculateResolutionState();
   void syncCameraViewport();
 
+  struct RenderTargetState {
+    uint32 framebuffer = 0;
+    int viewportX = 0;
+    int viewportY = 0;
+    int viewportWidth = 1;
+    int viewportHeight = 1;
+    float clearColor[4] = {0.0f, 0.0f, 0.0f, 0.0f};
+    Camera* camera = nullptr;
+    bool startedStandaloneBatch = false;
+  };
+
   ShaderManager shaderManager_;
   Batch batch_;
   Camera* camera_ = nullptr;
@@ -123,6 +144,8 @@ private:
   RenderResolutionState resolutionState_;
 
   bool initialized_ = false;
+  bool frameActive_ = false;
+  std::vector<RenderTargetState> renderTargetStack_;
 
   Renderer(const Renderer&) = delete;
   Renderer& operator=(const Renderer&) = delete;