sheet: Only notify the UI of progress after 32MB

This reduces the indexing time by 3-4x on a 13GB file with a Ryzen 7 6800U.

app/sheet/index.c

@@ -46,7 +46,12 @@ enum zsv_index_status build_memory_index(struct zsvsheet_index_opts *optsp) {
   optsp->uib->index = ixr.ix;
 
   char cancelled = 0;
+  size_t committed_bytes = 0;
   while (!cancelled && (zst = zsv_parse_more(ixr.parser)) == zsv_status_ok) {
+    if (zsv_cum_scanned_length(ixr.parser) - committed_bytes < 32 * 1024 * 1024)
+      continue;
+    committed_bytes = zsv_cum_scanned_length(ixr.parser);
+
     pthread_mutex_lock(&optsp->uib->mutex);
     if (optsp->uib->worker_cancelled) {
       cancelled = 1;

app/sheet/transformation.c

@@ -205,6 +205,7 @@ enum zsvsheet_status zsvsheet_push_transformation(zsvsheet_proc_context_t ctx,
   zopts.ctx = opts.user_context;
   zopts.row_handler = (void (*)(void *))opts.row_handler;
   zopts.stream = fopen(filename, "rb");
+  zopts.buffsize = 2 * 1024 * 1024;
 
   if (!zopts.stream)
     goto error;