BorderImage support for @2x assets

BorderImage support for @2x assets

Despite being a subclass of QQuickImageBase, BorderImage components did
not support using @2x assets like Image components.  The 9 patch image
logic now accounts for device pixel ratio when needed.

Manual tests added for stretch, repeat and round tiling modes.

[ChangeLog][BorderImage] Add support for @2x HiDPI border images.
This means, no more need to multiply the border sizes by the device
pixel ratio.

Change-Id: I79958739929964c816ba5dacedd9eaf93a60a183
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@theqtcompany.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>