Gets context from data display, not control display.

Hey, keydouble is just the thing I was looking for! For me, it didn't run, though; threw an X error:
```
X Error of failed request:  XRecordBadContext
  Major opcode of failed request:  128 (RECORD)
  Minor opcode of failed request:  5 (XRecordEnableContext)
  Context in failed request:  0x17
  Serial number of failed request:  10
  Current serial number in output stream:  10
```

It looks like you now have to get your initial context from the same display object that you use it with; I've made that change, and now it seems to work great for me. 

I have no idea if you still are actively using this, but here's the patch.

Thanks so much for writing this!