WebIDL: Use malloc/strcpy on Struct String setters, fix libass#77

From @TheOneric:
  For most libass APIs the char* strings remain owned by the caller with libass doing
  internal copies if needed. This means the caller must if necessary free the memory
  at some point after the libass API call finished. The track and event modification/creation
  APIs are different because they work close to library internals (which is also why their usage
  comes with more restrictions/obligations than other APIs and why they don't sensibly work with
  ass.h alone).

Since we transfer ownership of the pointer to libass, this is unlikely to cause memory leaks.

build/webidl_binder.py

@@ -2,6 +2,7 @@
 ## Patched to: 
 ##   - add integer pointers (IntPtr)
 ##   - implement ByteString type
+##   - fix string setter for structs
 ## From https://github.com/emscripten-core/emscripten/blob/f36f9fcaf83db93e6a6d0f0cdc47ab6379ade139/tools/webidl_binder.py
 
 # Copyright 2014 The Emscripten Authors.  All rights reserved.
@@ -723,7 +724,12 @@ def render_function(class_name, func_name, sigs, return_type, non_pointer,
       get_sigs = {0: []}
       set_sigs = {1: [Dummy({'type': m.type})]}
       get_call_content = take_addr_if_nonpointer(m) + 'self->' + attr
-      set_call_content = 'self->' + attr + ' = ' + deref_if_nonpointer(m) + 'arg0'
+      if m.type.isDOMString():
+        set_call_content = """char* copy = (char*)malloc(strlen(%s) + 1); 
+  strcpy(copy, %s);
+  self->%s = copy""" % (deref_if_nonpointer(m) + 'arg0', deref_if_nonpointer(m) + 'arg0', attr)
+      else:
+        set_call_content = 'self->' + attr + ' = ' + deref_if_nonpointer(m) + 'arg0'
 
     get_name = 'get_' + attr
     mid_js += [r'''