avcodec/hw_base_encode: restrict size of next_prev

Some drivers are more strict about the size of the reference lists given
(i.e. VAOn12 [1]). The next_prev list is used to handle multiple "L0"
references in AV1 encode. Restrict the size of next_prev based on the
value of ref_l0 when the GOP structure is initialized.

[1] https://github.com/intel/cartwheel-ffmpeg/issues/278

v2: fix indentation issues

libavcodec/hw_base_encode.c

@@ -190,12 +190,12 @@ static void hw_base_encode_add_next_prev(FFHWBaseEncodeContext *ctx,
         return;
     }
 
-    if (ctx->nb_next_prev < MAX_PICTURE_REFERENCES) {
+    if (ctx->nb_next_prev < ctx->ref_l0) {
         ctx->next_prev[ctx->nb_next_prev++] = pic;
         ++pic->ref_count[0];
     } else {
         --ctx->next_prev[0]->ref_count[0];
-        for (i = 0; i < MAX_PICTURE_REFERENCES - 1; i++)
+        for (i = 0; i < ctx->ref_l0 - 1; i++)
             ctx->next_prev[i] = ctx->next_prev[i + 1];
         ctx->next_prev[i] = pic;
         ++pic->ref_count[0];
@@ -662,6 +662,9 @@ int ff_hw_base_init_gop_structure(FFHWBaseEncodeContext *ctx, AVCodecContext *av
                                   uint32_t ref_l0, uint32_t ref_l1,
                                   int flags, int prediction_pre_only)
 {
+    ctx->ref_l0 = FFMIN(ref_l0, MAX_PICTURE_REFERENCES);
+    ctx->ref_l1 = FFMIN(ref_l1, MAX_PICTURE_REFERENCES);
+
     if (flags & FF_HW_FLAG_INTRA_ONLY || avctx->gop_size <= 1) {
         av_log(avctx, AV_LOG_VERBOSE, "Using intra frames only.\n");
         ctx->gop_size = 1;

libavcodec/hw_base_encode.h

@@ -193,6 +193,10 @@ typedef struct FFHWBaseEncodeContext {
     int end_of_stream;
     int p_to_gpb;
 
+    // The number of L0/L1 references supported by the driver.
+    int             ref_l0;
+    int             ref_l1;
+
     // Whether the driver supports ROI at all.
     int             roi_allowed;