basic rendermode inheritance introspection (no more hacky overlay list)

quadtree.py

@@ -34,6 +34,7 @@ from PIL import Image
 
 import nbt
 import chunk
+from c_overviewer import get_render_mode_inheritance
 from optimizeimages import optimize_image
 import composite
 
@@ -64,7 +65,7 @@ class QuadtreeGen(object):
         self.rendermode = rendermode
         
         # force png renderformat if we're using an overlay mode
-        if rendermode in chunk.overlay_rendermodes:
+        if 'overlay' in get_render_mode_inheritance(rendermode):
             self.imgformat = "png"
 
         # Make the destination dir