Drawing artifacts with floating point UI scale

The issue is created to track the discussed problems with floating point UI scale. See http://mail.openjdk.java.net/pipermail/2d-dev/2016-September/007718.html
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Hello,

JDK9 comes with HiDPI support on Windows/Linux which is really great. As
we gave it a try, we found it looking pretty good with an integer scale
(2x) but revealed some rendering flaws with float scales.

Let me please demonstrate it with SwingSet2 + JDK9-ea-b137 + Windows 8.1
in 150% scale (1.5f)

demo1 <http://cr.openjdk.java.net/%7Eant/hidpi_pics/demo1.png>

Dragging Frame-0 behind the pallet makes the pallet wavy.
Also, as Frame-0 moves it may leave traces.

demo2 <http://cr.openjdk.java.net/%7Eant/hidpi_pics/demo2.png>

Unstable look of a control. For instance, these two combos are decorated
differently (and not perfectly).

demo3 <http://cr.openjdk.java.net/%7Eant/hidpi_pics/demo3.png>

Scrolling traces.

demo4 <http://cr.openjdk.java.net/%7Eant/hidpi_pics/demo4.png>

Menu traces.
Colored rendering artifacts.

Additionally, I'm attaching a test source & pics kindly provided by
Renaud (cc'd) from AndroidStudio. The demo finely shows problems on the
example of primitive rendering.

Scaling 100%
<http://cr.openjdk.java.net/%7Eant/hidpi_pics/Scaling-100-percent.png>

Scaling 125%
<http://cr.openjdk.java.net/%7Eant/hidpi_pics/Scaling-125-percent.png>

Scaling 150%
<http://cr.openjdk.java.net/%7Eant/hidpi_pics/Scaling-150-percent.png>

It seems like most of the mentioned issues are caused by inaccurate
rounding performed during the rendering cycle.
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