lavc: add a sample_aspect_ratio field to AVFrame

The sample aspect ratio is a per-frame property, so it makes sense to
define it in AVFrame rather than in the codec/stream context.
Simplify application-level sample aspect ratio information extraction,
and allow further simplifications.

libavfilter/vsrc_buffer.h

@@ -31,10 +31,10 @@
 #include "avfilter.h"
 
 int av_vsrc_buffer_add_frame(AVFilterContext *buffer_filter, AVFrame *frame,
-                             int64_t pts, AVRational pixel_aspect);
+                             int64_t pts);
 
 int av_vsrc_buffer_add_frame2(AVFilterContext *buffer_filter, AVFrame *frame,
-                              int64_t pts, AVRational pixel_aspect, int width,
+                              int64_t pts, int width,
                               int height, enum PixelFormat  pix_fmt,
                               const char *sws_param);