Make logging callbacks library-safe

Make logging callbacks library-safe

Move the logging callbacks into the display context, rather than
having them as global state.  Add user-context parameter as well so
that users can distinguish between callbacks in different instances.

The default behaviour does not change, and LIBVA_MESSAGING_LEVEL
continues to be respected in that case.

Since we're breaking API here, also rename vaMessageCallback to
VAMessageCallback to be consistent with all other types.

Signed-off-by: Mark Thompson <sw@jkqxz.net>