avcodec/internal: Allow receive_frame codecs to use decode_simple pkt

Decoders implementing the receive_frame API currently mostly use
stack packets to temporarily hold the packet they receive from
ff_decode_get_packet(). This role directly parallels the role of
in_pkt, the spare packet used in decode_simple_internal for the
decoders implementing the traditional decoding API. Said packet
is unused by the generic code for the decoders implementing the
receive_frame API, so allow them to use it to fulfill the function
it already fulfills for the traditional API for both APIs.

There is only one caveat in this: The packet is automatically
unreferenced in avcodec_flush_buffers(). But this is actually
positive as it means the decoders don't have to do this themselves
(in case the packet is preserved between receive_frame calls).

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

libavcodec/internal.h

@@ -112,10 +112,6 @@
 #   define STRIDE_ALIGN 8
 #endif
 
-typedef struct DecodeSimpleContext {
-    AVPacket *in_pkt;
-} DecodeSimpleContext;
-
 typedef struct EncodeSimpleContext {
     AVFrame *in_frame;
 } EncodeSimpleContext;
@@ -137,7 +133,15 @@ typedef struct AVCodecInternal {
 
     void *thread_ctx;
 
-    DecodeSimpleContext ds;
+    /**
+     * This packet is used to hold the packet given to decoders
+     * implementing the .decode API; it is unused by the generic
+     * code for decoders implementing the .receive_frame API and
+     * may be freely used (but not freed) by them with the caveat
+     * that the packet will be unreferenced generically in
+     * avcodec_flush_buffers().
+     */
+    AVPacket *in_pkt;
     AVBSFContext *bsf;
 
     /**