Patch attached.  Adds GST_STRUCT_PADDING, GST_OBJECT_PADDING, and
GST_CLASS_PADDING to all public objects and classes, and most
non-trivial structures.  Padding size is currently 4 void pointers.

Created attachment 20515 [details] [review]
add padding

Hm, it looks good, but how do you want to extend this later? By using
unions? i.e.:

union {
  GST_WHATEVER_PADDING,
  actual_thing_goes_here
} ext;

? Or something else? Because if you make everything macros, actually
implementing the extensions later will still require us to either use
unions or to not use the macro. ;).

When something needs to be added, we need to replace the macro with
whatever the macro resolves as, and modify accordingly.

A union isn't a bad idea, since it's a way to always ensure that the
structure never changes size.  It doesn't really matter.  The point of
the macro is to actually put it in every structure.

Checked in.