Lots of scrolling fixes. Made scrolling work properly in not fully

Lots of scrolling fixes. Made scrolling work properly in not fully

2006-12-26  Kristian Rietveld  <kris@gtk.org>

	Lots of scrolling fixes.  Made scrolling work properly in not fully
	validated tree views; fixed a bunch of corner cases.

	* gtk/gtktreeview.c (validate_visible_area): if a row's dy is past
	upper - page_size, we know it is located at the end so the test for
	dy + height has been dropped.
	In the same case if area_below < 0, we know this is the last node
	in the tree view so area_above is page_size - height of this row.
	(validate_visible_area): got rid of subtracting new_height -
	old_height from area_{below,above}, it didn't make any sense at
	all and the full height of the row should be subtracted instead.
	(validate_visible_area): when scrolling to a given path, set the
	top row directly together with the dy offset, then sync that top_row
	to the dy.
	(gtk_tree_view_set_top_row): new function to directly set a top_row.
	(gtk_tree_view_dy_to_top_row): refactored to use
	gtk_tree_view_set_top_row().
	(gtk_tree_view_top_row_to_dy): make sure dy >= 0 when done.
	(gtk_tree_view_adjustment_changed): only update our dy and
	top_row if the adjustment's dy actually changed.