Issue #9366: Correct Rectangle Options Fixed Size toggle behavior

This patch attempts to fix part of issue 9366, regarding inconsistent toggle behavior for the "Fixed Size" dropdown in Rectangle Select options.

gimp_rectangle_options_fixed_rule_changed is called when either the checkbox or the dropdown for Fixed Size is changed. However, the "fixed-size-active" property was not updated until after it ran, so the toggle behavior for changing the sensitivity of the size field is inverted. This checks to see if the toggle was changed and updates the boolean before setting the size field as (in)sensitive.