libobs: Fix size mismatch warning

memcpy unlikely to exceed 4 GB, but just make VS happy.

libobs/obs-source.c

@@ -2600,12 +2600,13 @@ static inline void copy_frame_data_plane(struct obs_source_frame *dst,
 					 const struct obs_source_frame *src,
 					 uint32_t plane, uint32_t lines)
 {
-	if (dst->linesize[plane] != src->linesize[plane])
+	if (dst->linesize[plane] != src->linesize[plane]) {
 		for (uint32_t y = 0; y < lines; y++)
 			copy_frame_data_line(dst, src, plane, y);
-	else
+	} else {
 		memcpy(dst->data[plane], src->data[plane],
-		       dst->linesize[plane] * lines);
+		       (size_t)dst->linesize[plane] * (size_t)lines);
+	}
 }
 
 static void copy_frame_data(struct obs_source_frame *dst,