engine: expose sequence video dimensions in the timeline facade

- oakengine_sequence_get_video_params (width/height/pixel aspect) with
  assertions for the default 1920x1080 square-pixel sequence and the
  invalid-handle path
- oak-cli now renders at the sequence's real dimensions instead of the
  hardcoded 1920x1080

engine/include/oakengine/timeline.h

@@ -109,6 +109,14 @@ OAKENGINE_API int
 oakengine_sequence_get_frame_rate(const OakEngineSequence *self, int *num,
 								  int *den);
 
+/**
+ * @brief Sequence video dimensions and pixel aspect ratio
+ * (ViewerOutput::get_video_params()). Any output pointer may be NULL.
+ */
+OAKENGINE_API int
+oakengine_sequence_get_video_params(const OakEngineSequence *self, int *width,
+									int *height, int *par_num, int *par_den);
+
 /**
  * @brief Number of tracks per track type (Sequence::track_list(type)->
  * get_track_count()). Any of `video`/`audio`/`subtitle` may be NULL.