WebIDL: Workaround ensureCache in Struct Setters, fix libass#77

Currently the ensureCache create a temporary pointer that will
transfer the value to the WASM/C part and recycle it with to use in the
next value, but since the libass struct pointers are owned by the
library, the pointers can't be recycled or freed or can lead into
an undefined behavior.

This patch allocate new memory and copy the pointer before set into struct
attribute.

To avoid creating complex code, I decided to fix it in the webidl binder, but
i plan to make a way to mark if the attribute need to copy the pointer.

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'''