API: Add API to query width and height when rendering

API: Add API to query width and height when rendering

The functions are gtk_widget_get_allocated_width() and
gtk_widget_get_allocated_height().

They are currently identical to using width/height of
gtk_widget_get_allocation(), but are introduced for ease of use (no need
to use a custom struct) and to free people from having to think about
allocation.x and allocation.y (which is where the origin of the cairo
context in GtkWidget::draw is located).