Merge commit 'f25f5f8c62ec7728ee7f5dcc8f1abd0dc6235735' into release/2.2

* commit 'f25f5f8c62ec7728ee7f5dcc8f1abd0dc6235735':
  proresenc: Properly account for alpha plane

Conflicts:
	libavcodec/proresenc_kostya.c

See: 1578986a0da41ab417ddddf5964fa192d27b759f
Merged-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/proresenc_kostya.c

@@ -1254,7 +1254,7 @@ static av_cold int encode_init(AVCodecContext *avctx)
                                   + 200;
 
     if (ctx->alpha_bits) {
-         // alpha plane is run-coded and might run over bit budget
+         // The alpha plane is run-coded and might exceed the bit budget.
          ctx->frame_size_upper_bound += ctx->pictures_per_frame *
                                         ctx->slices_per_picture *
          /* num pixels per slice */     (ctx->mbs_per_slice * 256 *