Support setting the default window and pane background colours (window

and active pane via window-style and window-active-style options, an
individual pane by a new select-pane -P flag). From J Raynor.

window.c

@@ -705,6 +705,8 @@ window_pane_create(struct window *w, u_int sx, u_int sy, u_int hlimit)
 
 	wp->saved_grid = NULL;
 
+	memcpy(&wp->colgc, &grid_default_cell, sizeof wp->colgc);
+
 	screen_init(&wp->base, sx, sy, hlimit);
 	wp->screen = &wp->base;