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.
Port GstVaapiVideoPool, GstVaapiSurfacePool and GstVaapiImagePool to
GstVaapiMiniObject. Drop gst_vaapi_video_pool_get_caps() since it was
no longer used for a long time. Make object allocators static, i.e.
local to the shared library.
