grid: don't pre-allocate the entire grid (with all scrollback lines)

The row array may now contain NULL pointers. This means the
corresponding row hasn't yet been allocated and initialized.

On a resize, we explicitly allocate the visible rows.

Uninitialized rows are then allocated the first time they are
referenced.

terminal.h

@@ -94,11 +94,11 @@ struct damage {
 struct row {
     struct cell *cells;
     bool dirty;
-    bool initialized;
 };
 
 struct grid {
     int num_rows;
+    int num_cols;
     int offset;
     int view;