vf_zscale: Fix alpha destination graph for floating point pixel formats

This was setting the input pixel type instead of the output one,
leading to incorrect data being fed to the library.
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>

libavfilter/vf_zscale.c

@@ -615,7 +615,7 @@ static int filter_frame(AVFilterLink *link, AVFrame *in)
             s->alpha_dst_format.width = out->width;
             s->alpha_dst_format.height = out->height;
             s->alpha_dst_format.depth = odesc->comp[0].depth;
-            s->alpha_dst_format.pixel_type = (desc->flags & AV_PIX_FMT_FLAG_FLOAT) ? ZIMG_PIXEL_FLOAT : odesc->comp[0].depth > 8 ? ZIMG_PIXEL_WORD : ZIMG_PIXEL_BYTE;
+            s->alpha_dst_format.pixel_type = (odesc->flags & AV_PIX_FMT_FLAG_FLOAT) ? ZIMG_PIXEL_FLOAT : odesc->comp[0].depth > 8 ? ZIMG_PIXEL_WORD : ZIMG_PIXEL_BYTE;
             s->alpha_dst_format.color_family = ZIMG_COLOR_GREY;
 
             zimg_filter_graph_free(s->alpha_graph);