Make sure that libgstvaapi private headers remain internally used to
build libgstvaapi libraries only. All header dependencies were reviewed
and checks for IN_LIBGSTVAAPI definition were added accordingly.
Also rename GST_VAAPI_CORE definition to IN_LIBGSTVAAPI_CORE to keep
consistency.
The GLTextureUploadMeta implementation assumed that for each upload()
sequence, the supplied texture id is always the same as the one that
was previously cached into the underlying GstVaapiTexture. Cope with
any texture id change the expense to recreate the underlying VA/GLX
resources.
https://bugzilla.gnome.org/show_bug.cgi?id=719643
