Fix TUI source window drawing

The smaxrow and smaxcol parameters of prefresh are the bottom right corner
of the text area inclusive, not exclusive.

And if the source window grows bigger in height, the pad has to grow as
well.

gdb/ChangeLog:

2021-01-05  Hannes Domani  <ssbssa@yahoo.de>

	PR tui/26927
	* tui/tui-winsource.c (tui_source_window_base::refresh_window):
	Fix source pad size in prefresh.
	(tui_source_window_base::show_source_content): Grow source pad
	if necessary.