H.264/5: confirm visible_rectangle's semantics

[Gnurou]

While this methods returns a rectangle with min and max members, its max member seems to only take the width and height into account.

This ambiguity propagates to the visible_rect method of the VAAPI backend, which should probably use the same Rect type as these methods?

The computation of the visible width and height is performed in the visible_rectangle and in the parse_sps methods, using apparently slightly different methods. Need to verify with the spec which one is correct - also the pre-computed values in parse_sps are never used afterwards.