render: center grid in window

Instead of placing the upper left corner of the grid at 0,0 in the
window, center it.

terminal.h

@@ -274,6 +274,8 @@ struct terminal {
     int scale;
     int width;  /* pixels */
     int height; /* pixels */
+    int x_margin;
+    int y_margin;
     int cols;   /* number of columns */
     int rows;   /* number of rows */
     int cell_width;  /* pixels per cell, x-wise */