back/x11: demote the failed-offset-detection log to a debug log (#163)

-[XGServer(WindowOps) _checkStyle:] logs "Failed to determine offsets for
style" once per window style when the window manager does not reparent.
This is expected on rootless and exported servers and prints fifteen lines
on start.  The result is handled by guessing the offsets instead, so make
this a debug log, as was already done for the guessing message in 9299fa1.

Author: DTW-Thalion

Source/x11/XGServerWindow.m

@@ -1151,7 +1151,7 @@ - (BOOL) _checkStyle: (unsigned)style
     }
   if (o->known == NO)
     {
-      NSLog(@"Failed to determine offsets for style %d", style);
+      NSDebugLLog(@"Offset", @"Failed to determine offsets for style %d", style);
       return NO;
     }
   return YES;