upgrade to Filament 1.21.0

ios/polyvox_filament.podspec

@@ -21,6 +21,9 @@ A new flutter plugin project.
   s.static_framework = true
   s.vendored_libraries = "lib/*.a"
 
+  s.library = "c++"
+
+
   # Flutter.framework does not contain a i386 slice.
   s.user_target_xcconfig = {
     'CLANG_ALLOW_NON_MODULAR_INCLUDES_IN_FRAMEWORK_MODULES' => 'YES',