Cycles: detect when attributes have changed

This patch has originally been written by Kévin Dietrich, thanks!
It is part of D10210.

As Brecht noted in D10210, this might not handle all cases yet.
I better solution should come soonish.

intern/cycles/render/attribute.cpp

@@ -440,6 +440,7 @@ Attribute *AttributeSet::add(ustring name, TypeDesc type, AttributeElement eleme
 
   Attribute new_attr(name, type, element, geometry, prim);
   attributes.emplace_back(std::move(new_attr));
+  modified = true;
   return &attributes.back();
 }
 
@@ -461,6 +462,7 @@ void AttributeSet::remove(ustring name)
 
     for (it = attributes.begin(); it != attributes.end(); it++) {
       if (&*it == attr) {
+        modified = true;
         attributes.erase(it);
         return;
       }
@@ -606,6 +608,7 @@ void AttributeSet::remove(AttributeStandard std)
 
     for (it = attributes.begin(); it != attributes.end(); it++) {
       if (&*it == attr) {
+        modified = true;
         attributes.erase(it);
         return;
       }
@@ -671,12 +674,14 @@ void AttributeSet::update(AttributeSet &&new_attributes)
   for (it = attributes.begin(); it != attributes.end();) {
     if (it->std != ATTR_STD_NONE) {
       if (new_attributes.find(it->std) == nullptr) {
+        modified = true;
         attributes.erase(it++);
         continue;
       }
     }
     else if (it->name != "") {
       if (new_attributes.find(it->name) == nullptr) {
+        modified = true;
         attributes.erase(it++);
         continue;
       }
@@ -691,6 +696,7 @@ void AttributeSet::clear_modified()
   foreach (Attribute &attr, attributes) {
     attr.modified = false;
   }
+  modified = false;
 }
 
 /* AttributeRequest */