Increase accuracy of twips conversion

Interesting discovery: Screen.TwipsPerPixelX returns 7 at 200% DPI.  The
actual value should be 7.5, but due to integer truncation, 7 is used.
For precision UI positioning, this can cause big problems due to
rounding errors (especially if multiplied by some pixel amount, as PD
does frequently!).

I now wrap twips calculations with a TwipsPerPixelX/YFix function, which
returns 7.5 instead of 7 at 200% dpi.  This fixes a host of DPI issues
where controls were too short for their captions.