sys/xvimage/xvimagesink.c: Fix rounding errors when converting colorbalance values between hardware and object proper...

Original commit message from CVS:
* sys/xvimage/xvimagesink.c:
Fix rounding errors when converting colorbalance values
between hardware and object property ranges.  Partial
fix for #537889, however, there still seems to be a small
drift problem that could be totem's fault.