Fix duplicate DSpan exports

Arbitrary data from a material's layer flags were able to creep into a
DSpan's criteria, causing geometry that should go into the same span to
spawn a new DSpan with an identical name. Only one of the DSpan objects
would be exported, and Uru would either show crazy stuff or crash as a
result.

korman/exporter/mesh.py

@@ -70,7 +70,7 @@ class _RenderLevel:
 
 class _DrawableCriteria:
     def __init__(self, bo, hsgmat, pass_index):
-        self.blend_span = hsgmat.layers[0].object.state.blendFlags & hsGMatState.kBlendMask
+        self.blend_span = bool(hsgmat.layers[0].object.state.blendFlags & hsGMatState.kBlendMask)
         self.criteria = 0
 
         if self.blend_span: