Resources are reference-counted, if another object holds a reference to
a resource, it should not free it. Change the resource hashtable destroy
callback to unref.

Currently, this doesn't happen, since the resource hashtable is kept
until the renderer is reset. However, in the next commit, the destroy
callback will be used if a resource is replaced in the hashtable, and we
want to keep the original resource. Furthermore, even if replace
shouldn't happen, it avoids confusion and future errors if use only
reference-counting mechanism.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>