Add explicit casts to printing of function pointers (flutter#16370)

Compiling with clang on Windows fails here with a warning about implicit
cast from function pointer to object pointer. Rather than disable that
warning, this makes it an explicit cast. Since this is just test output,
it's not critical that this be a completely safe operation.

Part of flutter#16256

shell/platform/embedder/tests/embedder_assertions.h

@@ -224,16 +224,16 @@ inline std::ostream& operator<<(std::ostream& out,
                                 const FlutterOpenGLTexture& item) {
   return out << "(FlutterOpenGLTexture) Target: 0x" << std::hex << item.target
              << std::dec << " Name: " << item.name << " Format: " << item.format
-             << " User Data: " << item.user_data
-             << " Destruction Callback: " << item.destruction_callback;
+             << " User Data: " << item.user_data << " Destruction Callback: "
+             << reinterpret_cast<void*>(item.destruction_callback);
 }
 
 inline std::ostream& operator<<(std::ostream& out,
                                 const FlutterOpenGLFramebuffer& item) {
   return out << "(FlutterOpenGLFramebuffer) Target: 0x" << std::hex
              << item.target << std::dec << " Name: " << item.name
-             << " User Data: " << item.user_data
-             << " Destruction Callback: " << item.destruction_callback;
+             << " User Data: " << item.user_data << " Destruction Callback: "
+             << reinterpret_cast<void*>(item.destruction_callback);
 }
 
 inline std::string FlutterPlatformViewMutationTypeToString(
@@ -318,8 +318,8 @@ inline std::ostream& operator<<(std::ostream& out,
                                 const FlutterSoftwareBackingStore& item) {
   return out << "(FlutterSoftwareBackingStore) Allocation: " << item.allocation
              << " Row Bytes: " << item.row_bytes << " Height: " << item.height
-             << " User Data: " << item.user_data
-             << " Destruction Callback: " << item.destruction_callback;
+             << " User Data: " << item.user_data << " Destruction Callback: "
+             << reinterpret_cast<void*>(item.destruction_callback);
 }
 
 inline std::ostream& operator<<(std::ostream& out,