Fix rubberband selection outline position

Summary:
Before this patch, the outline of the rubberband selection control was down and to the right half a pixel too far.
This wasn't visible at 1x scaling because of the lack of antialiasing, but resulted in the top/left being too thin and bottom/right being too thick on high DPI displays.
To fix the issue, I enabled antialiasing and moved all sides inwards by half a pixel so that the outline would be pixel perfect.
I also added a function to get a QRectF with the correct size and position for rectangles with pen strokes.

Test Plan:
- Open dolphin and make a selection with the mouse
- Go to a Folder View Plasma widget and make a selection with the mouse

2x scaling
before: {F7818498, size=full}
after: {F7818500, size=full}

3x scaling
before: {F7818484}
after: {F7818483}

Reviewers: #vdg, #breeze, #plasma, hpereiradacosta, broulik

Reviewed By: hpereiradacosta

Subscribers: broulik, plasma-devel

Tags: #plasma, #breeze, #vdg

Differential Revision: https://phabricator.kde.org/D26001