lavu: introduce av_parse_ratio() and use it in ffmpeg and lavfi/aspect

Factorize code and provide ratio parsing consistency.

libavutil/log.h

@@ -124,6 +124,8 @@ typedef struct AVClass {
  */
 #define AV_LOG_DEBUG    48
 
+#define AV_LOG_MAX_OFFSET (AV_LOG_DEBUG - AV_LOG_QUIET)
+
 /**
  * Send the specified message to the log if the level is less than or equal
  * to the current av_log_level. By default, all logging messages are sent to