lavc: add intra only codec flag

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/avcodec.h

@@ -725,6 +725,10 @@ typedef struct RcOverride{
  * Audio encoder supports receiving a different number of samples in each call.
  */
 #define CODEC_CAP_VARIABLE_FRAME_SIZE 0x10000
+/**
+ * Codec is intra only.
+ */
+#define CODEC_CAP_INTRA_ONLY       0x40000000
 /**
  * Codec is lossless.
  */