Give all anonymously typedeffed structs in headers a name

Anonymous structs cannot be forward declared and have no benefit.

libavfilter/gradfun.h

@@ -25,7 +25,7 @@
 #include "avfilter.h"
 
 /// Holds instance-specific information for gradfun.
-typedef struct {
+typedef struct GradFunContext {
     int thresh;    ///< threshold for gradient algorithm
     int radius;    ///< blur radius
     int chroma_w;  ///< width of the chroma planes

libavfilter/yadif.h

@@ -22,7 +22,7 @@
 #include "libavutil/pixdesc.h"
 #include "avfilter.h"
 
-typedef struct {
+typedef struct YADIFContext {
     /**
      * 0: send 1 frame for each frame
      * 1: send 1 frame for each field