Fix bug in DiagramFrame.SetInfinite()

When calculating the center of the diagram, if prefs.centerDiagram is True, the values are currently float. They should be integer; change to:

```python

noUnitX = (vWidth-cWidth) // xUnit
noUnitY = (vHeight-cHeight) // yUnit

if self._prefs.centerDiagram is True:
self.Scroll(noUnitX // 2, noUnitY // 2) # set the scrollbars position in the middle of their scale
else:
self.Scroll(0, 0)

```