avfilter: Add a header for internal generic-layer APIs

This commit moves the generic-layer stuff (that is not used
by filters) to a new header of its own, similarly to
5e7b5b0090 for libavcodec.
thread.h and link_internal.h are merged into this header.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/graphparser.c

@@ -29,6 +29,7 @@
 #include "libavutil/opt.h"
 
 #include "avfilter.h"
+#include "avfilter_internal.h"
 #include "internal.h"
 
 #define WHITESPACES " \n\t\r"