Handle internal-border width changes when maximized. Bug #2568

* xterm.c (x_new_font): Move code for setting rows/cols before
resizing ...
(x_set_window_size): ... to here. bug #2568.

* gtkutil.c (xg_clear_under_internal_border): New function.
(xg_frame_resized, xg_frame_set_char_size): Call
xg_clear_under_internal_border.
(xg_update_scrollbar_pos): Clear under old scroll bar position.