Bump the stack size for WASAPI rendering threads.

In Gecko, webrtc.org code we call from the callback is using big arrays
allocated on the stack, and we're running out of stack space. Bumping it by a
factor of two should be enough.

src/cubeb_wasapi.cpp

@@ -1853,7 +1853,7 @@ int wasapi_stream_start(cubeb_stream * stm)
     return CUBEB_ERROR;
   }
 
-  stm->thread = (HANDLE) _beginthreadex(NULL, 256 * 1024, wasapi_stream_render_loop, stm, STACK_SIZE_PARAM_IS_A_RESERVATION, NULL);
+  stm->thread = (HANDLE) _beginthreadex(NULL, 512 * 1024, wasapi_stream_render_loop, stm, STACK_SIZE_PARAM_IS_A_RESERVATION, NULL);
   if (stm->thread == NULL) {
     LOG("could not create WASAPI render thread.\n");
     return CUBEB_ERROR;