This makes the trace output for arrays significantly nicer.

The type of the result of a shift operation is unaffected by the type of
the right operand, so some existing code overflows with undefined behaviour
when the element length is 32.  Add a helper macro to calculate the maximum
value correctly and then use it everywhere this pattern appears.
Found-by: Andreas Rheinhardt <andreas.rheinhardt@googlemail.com>

This makes it easier for users of the CBS API to get alloc/free right -
all subelements use the buffer API so that it's clear how to free them.
It also allows eliding some redundant copies: the packet -> fragment copy
disappears after this change if the input packet is refcounted, and more
codec-specific cases are now possible (but not included in this patch).

(cherry picked from commit 2bc9ba8d)
(cherry picked from commit a41b69b5)

(cherry picked from commit 867381b8)
(cherry picked from commit f7634893)
(cherry picked from commit 067a9dde)

(cherry picked from commit acf06f45)
(cherry picked from commit 768eb918)
(cherry picked from commit e7f64191)

(cherry picked from commit 18f1706f)
(cherry picked from commit 44cde38c)

Also enable MPEG-2 support in the trace_headers filter.