GstVaapiObject: initialize the derived object data with init() hook.

GstVaapiObject: initialize the derived object data with init() hook.

Call the subclass .init() function in gst_vaapi_object_new(), if
needed. The default behaviour is to zero initialize the subclass
object data, then the .init() function can be used to initialize
fields to non-default values, e.g. VA object ids to VA_INVALID_ID.

Also fix the gst_vaapi_object_new() description, which was merely
copied from GstVaapiMiniObject.

https://bugzilla.gnome.org/show_bug.cgi?id=722757

[changed to always zero initialize the subclass]
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>