vo_gpu_next: be explicit about screenshot repr

Screenshots are currently always RGB. Subject to change, but needs to be
communicated clearly if changed. This commit is not a functional change,
it's merely for code clarity.

video/out/vo_gpu_next.c

@@ -1217,6 +1217,7 @@ static void video_screenshot(struct vo *vo, struct voctrl_screenshot *args)
     }
 
     struct pl_frame target = {
+        .repr = pl_color_repr_rgb,
         .num_planes = 1,
         .planes[0] = {
             .texture = fbo,