video_output: convert HW pictures to RGBA for snapshot

If for some reason there's alpha data, it should also be found in the snapshot. Snapshots are stored as PNG by default, so it can handle RGBA sources.

src/video_output/video_output.c

@@ -1025,11 +1025,11 @@ static const struct filter_video_callbacks vout_video_cbs = {
     NULL, VoutHoldDecoderDevice,
 };
 
-static picture_t *ConvertRGB32AndBlend(vout_thread_sys_t *vout, picture_t *pic,
-                                     subpicture_t *subpic)
+static picture_t *ConvertRGBAAndBlend(vout_thread_sys_t *vout, picture_t *pic,
+                                      subpicture_t *subpic)
 {
     vout_thread_sys_t *sys = vout;
-    /* This function will convert the pic to RGB32 and blend the subpic to it.
+    /* This function will convert the pic to RGBA and blend the subpic to it.
      * The returned pic can't be used to display since the chroma will be
      * different than the "vout display" one, but it can be used for snapshots.
      * */
@@ -1046,8 +1046,7 @@ static picture_t *ConvertRGB32AndBlend(vout_thread_sys_t *vout, picture_t *pic,
 
     es_format_t src = sys->spu_blend->fmt_out;
     es_format_t dst = src;
-    dst.video.i_chroma = VLC_CODEC_RGB32;
-    video_format_FixRgb(&dst.video);
+    dst.video.i_chroma = VLC_CODEC_RGBA;
 
     filter_chain_Reset(filterc, &src,
                        NULL /* TODO output video context of blender */,
@@ -1221,7 +1220,7 @@ static int PrerenderPicture(vout_thread_sys_t *sys, picture_t *filtered,
                      * software RGB32 to generate a snapshot. */
                     if (do_snapshot)
                     {
-                        picture_t *copy = ConvertRGB32AndBlend(sys, blent, subpic);
+                        picture_t *copy = ConvertRGBAAndBlend(sys, blent, subpic);
                         if (copy)
                             snap_pic = copy;
                     }