Since we started depending on GLib 2.44, we can be sure this macro is
defined (it will be a no-op on compilers that don't support it). For
plugins we should just start using `G_DECLARE_FINAL_TYPE` which means
we no longer need the macro there, but for most types in core we don't
want to break ABI, which means it's better to just keep it like it is
(and use the `#ifdef` instead).
We need different export decorators for the different libs.
For now no actual change though, just rename before the release,
and add prelude headers to define the new decorator to GST_EXPORT.
This is equal to any other caps features but results in unfixed caps. It
would be used by elements that only look at the buffer metadata or are
currently working in passthrough mode, and as such don't care about any
specific features.
These are meant to specify features in caps that are required
for a specific structure, for example a specific memory type
or meta.
Semantically they could be though of as an extension of the media
type name of the structures and are handled exactly like that.
