avcodec/cbs: allow cbs_read_fragment_content() to skip decomposition of units

The caller may not need all units in a fragment in reading only scenarios.
They could in fact alter global state stored in the private CodedBitstreamType
fields in an undesirable way.
With this change, unit decomposition can be skipped based on parsed values
within the unit.

Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/cbs.c

@@ -202,6 +202,12 @@ static int cbs_read_fragment_content(CodedBitstreamContext *ctx,
             av_log(ctx->log_ctx, AV_LOG_VERBOSE,
                    "Decomposition unimplemented for unit %d "
                    "(type %"PRIu32").\n", i, unit->type);
+        } else if (err == AVERROR(EAGAIN)) {
+            av_log(ctx->log_ctx, AV_LOG_VERBOSE,
+                   "Skipping decomposition of unit %d "
+                   "(type %"PRIu32").\n", i, unit->type);
+            av_buffer_unref(&unit->content_ref);
+            unit->content = NULL;
         } else if (err < 0) {
             av_log(ctx->log_ctx, AV_LOG_ERROR, "Failed to read unit %d "
                    "(type %"PRIu32").\n", i, unit->type);