vd_lavc: let hardware decoder request delaying frames explicitly

Until now, the presence of the process_image() callback was used to set
a delay queue with a hardcoded size. Change this to a vd_lavc_hwdec
field instead, so the decoder can explicitly set this if it's really
needed.

Do this so process_image() can be used in the VideoToolbox glue code for
something entirely unrelated.

video/decode/d3d11va.c

@@ -495,4 +495,5 @@ const struct vd_lavc_hwdec mp_vd_lavc_d3d11va_copy = {
     .init_decoder   = d3d11va_init_decoder,
     .allocate_image = d3d11va_allocate_image,
     .process_image  = d3d11va_retrieve_image,
+    .delay_queue    = HWDEC_DELAY_QUEUE_COUNT,
 };

video/decode/dxva2.c

@@ -516,4 +516,5 @@ const struct vd_lavc_hwdec mp_vd_lavc_dxva2_copy = {
     .init_decoder   = dxva2_init_decoder,
     .allocate_image = dxva2_allocate_image,
     .process_image  = dxva2_retrieve_image,
+    .delay_queue    = HWDEC_DELAY_QUEUE_COUNT,
 };

video/decode/lavc.h

@@ -49,6 +49,11 @@ struct vd_lavc_hwdec {
     // If not-0: the IMGFMT_ format that should be accepted in the libavcodec
     // get_format callback.
     int image_format;
+    // Setting this will queue the given number of frames before calling
+    // process_image() or returning them to the renderer. This can increase
+    // efficiency by not blocking on the hardware pipeline by reading back
+    // immediately after decoding.
+    int delay_queue;
     int (*probe)(struct vd_lavc_hwdec *hwdec, struct mp_hwdec_info *info,
                  const char *codec);
     int (*init)(struct lavc_ctx *ctx);

video/decode/vaapi.c

@@ -508,4 +508,5 @@ const struct vd_lavc_hwdec mp_vd_lavc_vaapi_copy = {
     .init_decoder = init_decoder,
     .allocate_image = allocate_image,
     .process_image = copy_image,
+    .delay_queue = HWDEC_DELAY_QUEUE_COUNT,
 };

video/decode/vd_lavc.c

@@ -401,10 +401,7 @@ static void init_avctx(struct dec_video *vd, const char *decoder,
             avctx->get_buffer2 = get_buffer2_hwdec;
         if (ctx->hwdec->init(ctx) < 0)
             goto error;
-        // This can increase efficiency by not blocking on the hardware
-        // pipeline by reading back immediately after decoding.
-        if (ctx->hwdec->process_image)
-            ctx->max_delay_queue = HWDEC_DELAY_QUEUE_COUNT;
+        ctx->max_delay_queue = ctx->hwdec->delay_queue;
     } else {
         mp_set_avcodec_threads(vd->log, avctx, lavc_param->threads);
     }