Avoid rounding errors in size calculation.

[5E-324]

Width and height are normally fractional when DPI scaling is enabled, converting them to int will introduce rounding errors, and sometimes produces white lines on the right and bottom of the control. To avoid this, they are now stored as double, the same as in UIElement.RenderSize.

[NogginBops]

@5E-324 I'm guessing it wasn't intentional to close the repo by deleting the repo? Going to reopen this, we want to fix DPI for this control soon.

[NogginBops]

Closing if favor of #86