Mark wxGLCanvas::DefaultSwapInterval read-only explicitly. SIP does not always
infer this from the const-qualified Doxygen type and otherwise generates a
setter that cannot compile.

--- a/etg/_glcanvas.py
+++ b/etg/_glcanvas.py
@@ -86,6 +86,7 @@ def run():
     c.find('GetGLAttrs').ignore()
 
     c = module.find('wxGLCanvas')
     tools.fixWindowClass(c)
+    c.find('DefaultSwapInterval').noSetter = True
 
     # We already have a MappedType for wxArrayInt, so just tweak the