Track ImageAssets so we can delete them

include/rive/file.hpp

diff --git a/include/rive/file.hpp b/include/rive/file.hpp
index c7b4017..9d2ea3f 100644
--- a/include/rive/file.hpp
+++ b/include/rive/file.hpp
@@ -42,6 +42,9 @@
         /// where the artboards live.
         std::unique_ptr<Backboard> m_Backboard;
 
+        /// We just keep these alive for the life of this File
+        std::vector<std::unique_ptr<FileAsset>> m_FileAssets;
+
         /// List of artboards in the file. Each artboard encapsulates a set of
         /// Rive components and animations.
         std::vector<std::unique_ptr<Artboard>> m_Artboards;

src/file.cpp

diff --git a/src/file.cpp b/src/file.cpp
index a231f9c..27266a8 100644
--- a/src/file.cpp
+++ b/src/file.cpp
@@ -168,6 +168,10 @@
                     ab->m_Factory = m_Factory;
                     m_Artboards.push_back(std::unique_ptr<Artboard>(ab));
                 } break;
+                case ImageAsset::typeKey: {
+                    auto fa = object->as<FileAsset>();
+                    m_FileAssets.push_back(std::unique_ptr<FileAsset>(fa));
+                } break;
             }
         } else {
             fprintf(stderr, "Failed to import object of type %d\n", object->coreType());