The previous port to the draw function was a tiny bit incomplete. This
patch should fix the remaining issues and remove unused variables.

This commit add some text about mult-window ::draw implementations,
pointing out the gtk_cairo_should_draw_window() and
gtk_cairo_transform_to_window() convenience functions.

The function reverses the transform that GTK does before emitting a draw
event. So we can use it in "old" widgets to revert the coordinate system
properly.

This mostly goes to keep consistency with the changes to GtkSizeRequest
in the last patch, as GtkCellSizeRequest requires GtkCellRenderer and
GtkCellRenderer implements GtkCellSizeRequest there's no use in keeping
them separate.

This patch renames the functions:
gtk_cell_size_request_get_request_mode()
  => gtk_cell_renderer_get_request_mode()
gtk_cell_size_request_get_width()
  => gtk_cell_renderer_get_preferred_width()
gtk_cell_size_request_get_height()
  => gtk_cell_renderer_get_preferred_height()
gtk_cell_size_request_get_size()
  => gtk_cell_renderer_get_preferred_size()
gtk_cell_size_request_get_width_for_height()
  => gtk_cell_renderer_get_preferred_width_for_height()
gtk_cell_size_request_get_height_for_width()
  => gtk_cell_renderer_get_preferred_height_for_width()
... and moves the corresponding vfuncs to GtkCellRenderer.

The patch also renames the implementations of these functions in cell
renderers to include the word "preferrred".

It doesn't make sense to keep them separate as GtkSizeRequest requires a
GtkWidget and GtkWidget implements GtkSizeRequest, so you can never have
one without the other.
It also makes the code a lot easier because no casts are required when
calling functions.

Also, the names would translate to gtk_widget_get_width() and people
agreed that this would be a too generic name, so a "preferred" was added
to the names.

So this patch moves the functions:
gtk_size_request_get_request_mode() => gtk_widget_get_request_mode()
gtk_size_request_get_width() => gtk_widget_get_preferred_width()
gtk_size_request_get_height() => gtk_widget_get_preferred_height()
gtk_size_request_get_size() => gtk_widget_get_preferred_size()
gtk_size_request_get_width_for_height() =>
  gtk_widget_get_preferred_width_for_height()
gtk_size_request_get_height_for_width() =>
  gtk_widget_get_preferred_height_for_width()
... and moves the corresponding vfuncs to the GtkWidgetClass.

The patch also renames the implementations of the vfuncs in widgets to
include the word "preferrred".

This should only be used on windows anyway and GdkWindow has an
implementation for both functions.