Fix geometry.rectmidpoint() mathematical error.

With a rect 
```
{
   h = 1058,
   w = 1920,
   x = 0,
   y = 1080
}
```

You'd expect `geometry.rectmidpoint()` to return:
```
{
  x =  960,
  y = -551
}
```

However, it returned:
```
{
   x = 960,
   y = 1609
}
```

This fixes that issue.