[NbtkTable] Clamp the child box to pixel boundaries
[NbtkTable] Clamp the child box to pixel boundaries

When allocating a box we must always clamp to whole pixels rather than
fractional units. If we don't do this ClutterText ends up drawing wobbly text.

jonnylamb/mx,clutter-project/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,pwithnall/mx,clutter-project/mx,pwithnall/mx,pwithnall/mx,jonnylamb/mx,clutter-project/mx