Drop obsolete GST_VAAPI_IS_xxx() helper macros since we are no longer
deriving from GObject and so those were only checking for whether the
argument was NULL or not. This is now irrelevant, and even confusing
to some extent, because we no longer have type checking.
Note: this incurs more type checking (review) but the libgstvaapi is
rather small, so this is manageable.
Add more attributes for raw decoding modes, i.e. directly through the
libgstvaapi helper library. In particular, add presentation timestamp,
duration and a couple of flags (interlaced, TFF, RFF, one-field).
