Fix UIFrame RenderTexture positioning and toggling issues

- Fix #223: Use `position` instead of `box.getPosition()` for render_sprite
  positioning. The box was being set to (0,0) for texture rendering and
  never restored, causing frames to render at wrong positions.

- Fix #224: Add disableRenderTexture() method and call it when toggling
  clip_children or cache_subtree off. This properly cleans up the texture
  and prevents stale rendering.

- Fix #225: Improve dirty propagation in markContentDirty() to propagate
  to parent even when already dirty, if the parent was cleared (rendered)
  since last propagation. Prevents child changes from being invisible.

- Fix #226: Add fallback to standard rendering when RenderTexture can't
  be created (e.g., zero-size frame). Prevents inconsistent state.

Closes #223, closes #224, closes #225, closes #226

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/UIDrawable.h

@@ -248,6 +248,12 @@ protected:
     // Enable RenderTexture for this drawable
     void enableRenderTexture(unsigned int width, unsigned int height);
     void updateRenderTexture();
+
+    // Disable RenderTexture for this drawable (public for property setters)
+public:
+    void disableRenderTexture();
+
+protected:
     
 public:
     // #144: Dirty flag system - content vs composite