Functions: support cycles in lazy-function graph

Lazy-function graphs are now evaluated properly even if they contain
cycles. Note that cycles are only ok if there is no data dependency cycle.
For example, a node might output something that is fed back into itself.
As long as the output can be computed without the input that it feeds into,
everything is ok.

The code that builds the graph is responsible for making sure that there
are no actual data dependencies.

source/blender/functions/intern/lazy_function.cc

@@ -36,8 +36,22 @@ void LazyFunction::destruct_storage(void *storage) const
   UNUSED_VARS_NDEBUG(storage);
 }
 
+void LazyFunction::possible_output_dependencies(const int /*output_index*/,
+                                                const FunctionRef<void(Span<int>)> fn) const
+{
+  /* The output depends on all inputs by default. */
+  Vector<int, 16> indices(inputs_.size());
+  for (const int i : inputs_.index_range()) {
+    indices[i] = i;
+  }
+  fn(indices);
+}
+
 bool LazyFunction::always_used_inputs_available(const Params &params) const
 {
+  if (allow_missing_requested_inputs_) {
+    return true;
+  }
   for (const int i : inputs_.index_range()) {
     const Input &fn_input = inputs_[i];
     if (fn_input.usage == ValueUsage::Used) {