!sdf node is an implicit union so make it accept same attributes

Author: jonathanhogg

docs/canvas3d.md

@@ -1100,8 +1100,12 @@ to alter the boundaries between the combined surfaces (or with trim plane):
 `chamfer=` *DISTANCE*
 : An inset/outset distance for a 45° chamfer.
 
-The `!sdf` node also supports providing a custom SDF function with the following
-attribute:
+An `!sdf` node with multiple children represents an implicit `!union` operation
+and so supports the same `smooth`, `fillet` and `chamfer` attributes.
+
+Instead of providing a tree of primitive models and operations, an `!sdf` node
+may also be specified as a custom signed distance field function by providing
+the following attribute:
 
 `function=` *FUNCTION*
 : A **Flitter** function taking a 3-vector position (in pre-transform model

src/flitter/render/window/canvas3d.pyx

@@ -476,7 +476,10 @@ cdef Model get_model(Node node, bint top):
                 function.objects is not None and callable(f := function.objects[0]):
             model = Model._sdf(f, None, minimum, maximum, resolution)
         else:
-            model = Model._boolean('union', [get_model(child, False) for child in node._children], 0, 0, 0)
+            model = Model._boolean('union', [get_model(child, False) for child in node._children],
+                                   node.get_float('smooth', 0),
+                                   node.get_float('fillet', 0),
+                                   node.get_float('chamfer', 0))
             model = Model._sdf(None, model, minimum, maximum, resolution)
     elif node.kind is 'mix':
         model = Model._mix([get_model(child, False) for child in node._children], node.get_fvec('weights', 0, true_))