avcodec/h264_redundant_pps_bsf: Support multiple input PPS

Up until now, the h264_redundant_pps_bsf stored the initial value
of pic_init_qp_minus26 of the most recently encountered PPS;
it also modified the slices based upon to assumption that
the most recent PPS is the PPS the slice belongs to.
Yet this assumption is flawed, as there can be several PPS
with different IDs that are visible at any given time.
If these have different pic_init_qp_minus26 values,
the output can be invalid.

Fix this by directly using the pic_init_qp_minus26 value of
the input PPS.

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

libavcodec/h264_redundant_pps_bsf.c

@@ -32,7 +32,6 @@ typedef struct H264RedundantPPSContext {
     CBSBSFContext common;
 
     int global_pic_init_qp;
-    int current_pic_init_qp;
 } H264RedundantPPSContext;
 
 
@@ -50,9 +49,7 @@ static int h264_redundant_pps_fixup_pps(H264RedundantPPSContext *ctx,
         return err;
     pps = unit->content;
 
-    // Record the current value of pic_init_qp in order to fix up
-    // following slices, then overwrite with the global value.
-    ctx->current_pic_init_qp = pps->pic_init_qp_minus26 + 26;
+    // Overwrite pic_init_qp with the global value.
     pps->pic_init_qp_minus26 = ctx->global_pic_init_qp - 26;
 
     // Some PPSs have this set, so it must be set in all of them.
@@ -66,10 +63,13 @@ static int h264_redundant_pps_fixup_pps(H264RedundantPPSContext *ctx,
 static int h264_redundant_pps_fixup_slice(H264RedundantPPSContext *ctx,
                                           H264RawSliceHeader *slice)
 {
-    int qp;
+    const CodedBitstreamH264Context *const in = ctx->common.input->priv_data;
+    const H264RawPPS *const pps = in->pps[slice->pic_parameter_set_id];
 
-    qp = ctx->current_pic_init_qp + slice->slice_qp_delta;
-    slice->slice_qp_delta = qp - ctx->global_pic_init_qp;
+    // We modified the PPS's qp value, now offset this by applying
+    // the negative offset to the slices.
+    slice->slice_qp_delta += pps->pic_init_qp_minus26
+                             - (ctx->global_pic_init_qp - 26);
 
     return 0;
 }