This structure is always allocated by GStreamer, can't be
subclassed or extended, and is never allocated or used on
the stack, so we don't need any padding and can extend it
as we please.
These account for both possible type size mismatch AND -mms-bitfields
packing. Sizes are taken from an i686-w64-mingw32-built GStreamer,
gcc 4.8.0, mingw-w64 svn-r5685.
Fixes#697551