hwcontext: Add frame context mapping for nontrivial contexts

Some frames contexts are not usable without additional format-specific
state in hwctx.  This change adds new functions frames_derive_from and
frames_derive_to to initialise this state appropriately when deriving
a frames context which will require it to be set.

libavutil/hwcontext.c

@@ -816,7 +816,14 @@ int av_hwframe_ctx_create_derived(AVBufferRef **derived_frame_ctx,
         goto fail;
     }
 
-    ret = av_hwframe_ctx_init(dst_ref);
+    ret = AVERROR(ENOSYS);
+    if (src->internal->hw_type->frames_derive_from)
+        ret = src->internal->hw_type->frames_derive_from(dst, src, flags);
+    if (ret == AVERROR(ENOSYS) &&
+        dst->internal->hw_type->frames_derive_to)
+        ret = dst->internal->hw_type->frames_derive_to(dst, src, flags);
+    if (ret == AVERROR(ENOSYS))
+        ret = 0;
     if (ret)
         goto fail;
 

libavutil/hwcontext_internal.h

@@ -92,6 +92,11 @@ typedef struct HWContextType {
                                const AVFrame *src, int flags);
     int              (*map_from)(AVHWFramesContext *ctx, AVFrame *dst,
                                  const AVFrame *src, int flags);
+
+    int              (*frames_derive_to)(AVHWFramesContext *dst_ctx,
+                                         AVHWFramesContext *src_ctx, int flags);
+    int              (*frames_derive_from)(AVHWFramesContext *dst_ctx,
+                                           AVHWFramesContext *src_ctx, int flags);
 } HWContextType;
 
 struct AVHWDeviceInternal {