tui-winsource: Allocate for actual lines only

tui-winsource: Allocate for actual lines only

The logic for allocating a TUI source window's content buffer allocates
two more lines than needed, because it does not reduce the window height
by the highlight box's overhead.  However, it does reduce the line width
accordingly.  This patch makes the height and width calculation
consistent and improves the comment.

gdb/ChangeLog:

	* tui/tui-winsource.c (tui_alloc_source_buffer): Subtract
	highlight box's overhead when calculating the content height.