vdpau: don't use a transparent image format for screenshots

Fixes #1779.

video/vdpau.c

@@ -70,7 +70,7 @@ static struct mp_image *download_image(struct mp_hwdec_ctx *hwctx,
                               mpi, &in) < 0)
         goto error;
 
-    res = mp_image_pool_get(swpool, IMGFMT_BGR32, ctx->getimg_w, ctx->getimg_h);
+    res = mp_image_pool_get(swpool, IMGFMT_BGR0, ctx->getimg_w, ctx->getimg_h);
     if (!res)
         goto error;