pad: add ACCEPT_INTERCEPT flag

pad: add ACCEPT_INTERCEPT flag

Make a new flag on the pad that tweaks the default behaviour of the
accept-caps function. By default it will check for a subset of the
query-caps result but this is not always desirable. The query-caps
result contains all the constraints to make a good caps decision
upstream but sometimes, like for parsers, not all the constrained caps
fields are known upstream and then a subset check would fail. Switching
to an intersection makes this work again.

See https://bugzilla.gnome.org/show_bug.cgi?id=705024
    https://bugzilla.gnome.org/show_bug.cgi?id=677401