Change Seat's get_char_cell_size to get_window_pixel_size.

That's more directly useful in uxpty.c (which is currently the only
actual client of the function), and also matches the data that SSH
clients send in "pty-req". Also, it makes that method behave more like
the GUI query function get_window_pixels used by terminal.c (with the
sole exception that unlike g_w_p it's allowed to return failure), so
it becomes even more trivial to implement in the GUI front ends.

unix/uxplink.c

@@ -394,7 +394,7 @@ static const SeatVtable plink_seat_vt = {
     plink_echoedit_update,
     nullseat_get_x_display,
     nullseat_get_windowid,
-    nullseat_get_char_cell_size,
+    nullseat_get_window_pixel_size,
 };
 static Seat plink_seat[1] = {{ &plink_seat_vt }};