Don't deref the sample pointer in the wl_container_of macro

The previous implementation of the wl_container_of macro was
dereferencing the sample pointer in order to get an address of the
member to calculate the offset. Ideally this shouldn't cause any
problems because the dereference doesn't actually cause the address to
be read from so it shouldn't matter if the pointer is uninitialised.
However this is probably technically invalid and could cause undefined
behavior. Clang appears to take advantage of this undefined behavior
and doesn't bother doing the subtraction. It also gives a warning when
it does this.

The documentation for wl_container_of implies that it should only be
given an initialised pointer and if that is done then there is no
problem with clang. However this is quite easy to forget and doesn't
cause any problems or warnings with gcc so it's quite easy to
accidentally break clang.

To fix the problem this changes the macro to use pointer -
offsetof(__typeof__(sample), member) so that it doesn't need to deref
the sample pointer. This does however require that the __typeof__
operator is supported by the compiler. In practice we probably only
care about gcc and clang and both of these happily support the
operator.

The previous implementation was also using __typeof__ but it had a
fallback path avoiding it when the operator isn't available. The
fallback effectively has undefined behaviour and it is targetting
unknown compilers so it is probably not a good idea to leave it in.
Instead, this patch just removes it. If someone finds a compiler that
doesn't have __typeof__ but does work with the old implementation then
maybe they could add it back in as a special case.

This patch removes the initialisation anywhere where the sample
pointer was being unitialised before using wl_container_of. The
documentation for the macro has also been updated to specify that this
is OK.