Refactor|libdeng2|RuleRectangle: Reimplemented based on IndirectRule

RuleRectangle now defines the output rules directly in terms of
inputs, however the outputs are indirect so that anyone depending on
them does not get distracted by the possible changing of inputs
later on.

This results in a substantially more elegant implementation for
RuleRectangle that no longer has to rely on crutches to operate
correctly: each output rule knows exactly how it depends on its
inputs, so rule invalidation can work normally.

Furthermore, the rectangle's anchor point animation can now be done
with scalar rules without need for any extra work.

The clunky DelegateRule was removed entirely.

doomsday/libdeng2/include/de/widgets/indirectrule.h

@@ -45,6 +45,8 @@ class IndirectRule : public Rule
      */
     void setSource(Rule const &rule);
 
+    void unsetSource();
+
     void update();
 
     Rule const &source() const;

doomsday/libdeng2/include/de/widgets/rules.h

@@ -19,7 +19,6 @@
 
 #include "../Rule"
 #include "../ConstantRule"
-#include "../DelegateRule"
 #include "../IndirectRule"
 #include "../OperatorRule"
 #include "../ScalarRule"

doomsday/libdeng2/src/widgets/indirectrule.cpp

@@ -25,22 +25,27 @@ IndirectRule::IndirectRule() : _source(0)
 
 IndirectRule::~IndirectRule()
 {
-    independentOf(_source);
+    unsetSource();
 }
 
 void IndirectRule::setSource(Rule const &rule)
 {
-    if(_source)
-    {
-        independentOf(_source);
-    }
+    unsetSource();
     dependsOn(_source = &rule);
 
     invalidate();
 }
 
+void IndirectRule::unsetSource()
+{
+    independentOf(_source);
+    _source = 0;
+}
+
 void IndirectRule::update()
 {
+    DENG2_ASSERT(_source != 0);
+
     setValue(_source? _source->value() : 0);
 }