Fixed get_size() for GtkCellRendererText to clip to the input area

Fixed get_size() for GtkCellRendererText to clip to the input area

For ellipsize cells it's important to clip the result of get_size()
so that the returned required rectangle is indeed less than or equal
to the input rectangle... this is done so that GtkCellArea can accurately
paint focus on cells by calling gtk_cell_renderer_get_aligned_area().

Patch also adds assertions to gtk_cell_renderer_get_aligned_area() to
ensure this keeps working correctly.