...that of DPIUtil#autoScaleImageData

- In FileImageDescriptor#getxPath replace truncation rounding by
Math.round following the style used in SWT's DPIUtil class
- Add additional test case for 150% zoom, testing both expected height
and width
- Extend existing testGetxPathRectangular() to test also the expected
height

Signed-off-by: Daniel Krügler <daniel.kruegler@gmail.com>