lavu: bump minor and update APIchanges for the new transform API

doc/APIchanges

@@ -15,6 +15,9 @@ libavutil:     2017-10-21
 
 API changes, most recent first:
 
+2019-05-15 - b79b29ddb1 - lavu 56.28.100 - tx.h
+  Add av_tx_init(), av_tx_uninit() and related definitions.
+
 2019-04-20 - 3153a6502a - lavc 58.52.100 - avcodec.h
   Add AV_CODEC_FLAG_DROPCHANGED to allow avcodec_receive_frame to drop
   frames whose parameters differ from first decoded frame in stream.

libavutil/version.h

@@ -79,7 +79,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR  56
-#define LIBAVUTIL_VERSION_MINOR  27
+#define LIBAVUTIL_VERSION_MINOR  28
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \