core: restore --mc default value (fixes A/V sync)

Commit bc20f2c moved the variable default_max_pts_correction (which
backs the --mc option) to the MPOpts struct. The initializer was
forgotten when doing this, so it was left at 0. This disabled part of
the A/V sync mechanism. This was apparent when using ad_spdif (this
decoder has other A/V sync related problems, and thus triggers this
issue easily).

Closes #59.

core/defaultopts.c

@@ -56,6 +56,7 @@ void set_default_mplayer_options(struct MPOpts *opts)
         .stream_cache_pause = 10.0,
         .chapterrange = {-1, -1},
         .edition_id = -1,
+        .default_max_pts_correction = -1,
         .user_correct_pts = -1,
         .initial_audio_sync = 1,
         .term_osd = 2,