This is a fix of the antialiasing logic of FlatStyleBox.
It is now possible to have smooth rounded corners while keeping
the edges sharp on the pixels.
The antialiasing gradient positioning is ajusted so that the "hard"
border corresponds to the middle of that gradient instead of one end.
Checked against rendering of rounded rectangles in a vector graphics
software.
(cherry picked from commit 2ef20045b1)
75f3ce618e