The hierarchy lines of the outliner are currently always one pixel wide, which can be hard to see well on monitors of high pixel density.
This patch makes it so that the lines are draw in a width of U.pixelsize which changes with user scale and user line width.
So at 1X scale the lines are drawn as one pixel wide, as it is now. But at 2X scale they will be drawn 2 pixels wide by default (auto line width), or at 1 pixel if the user selects "thin" lines, or 3 if selecting "thick" lines.
Following shows the line width at 2X scale and auto (2-pixel) line width.