Add loading screen with random WOWEE splash images

- Add loading screen system with stb_image for JPEG loading
- Two loading screen images (orc and dwarf) randomly selected
- Display loading screen while terrain data loads
- Cache WMO inverse matrices to reduce per-frame computation
- Stub WMO liquid rendering (needs coordinate system fix)
- Update spawn point to Stormwind Trade District

include/rendering/wmo_renderer.hpp

@@ -221,6 +221,7 @@ private:
         glm::vec3 rotation;  // Euler angles (radians)
         float scale;
         glm::mat4 modelMatrix;
+        glm::mat4 invModelMatrix;  // Cached inverse for collision
 
         void updateModelMatrix();
     };