Subpixel rendering: Rounded rect gets non-renderable at certain subpixel size.
https://bugs.webkit.org/show_bug.cgi?id=135314
<rdar://problem/17812921>

Reviewed by Tim Horton.

While calculating the rounded rect for painting, the radius is adjusted to compensate
for the pixel snapped size. However while scaling the radius, certain values overflow
(float) mantissa and it produces a non-renderable rounded rect where the radius becomes bigger
than the rectangle dimensions. In such cases, we need to shrink the radius to make it
renderable again.

Source/WebCore:
Test: transitions/rounded-rect-becomes-non-renderable-while-transitioning.html

* platform/graphics/RoundedRect.cpp:
(WebCore::RoundedRect::pixelSnappedRoundedRectForPainting): shrink the radius by
one device pixel. It is as good as any other small value.

LayoutTests:
* transitions/rounded-rect-becomes-non-renderable-while-transitioning-expected.txt: Added.
* transitions/rounded-rect-becomes-non-renderable-while-transitioning.html: Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@171640 268f45cc-cd09-0410-ab3c-d52691b4dbfc