WinRT: Implement Direct2D backend

For Win10 IoT Core/Pro we'd like to get rid of ANGLE. Many boards do not support feature level 9.3 and hence ANGLE jumps back to WARP, causing slow rendering times.

Instead we'd like to use D2D in the platform plugin, combined with the software renderer available in 5.8 and later, as that one has partial updates incorporated, which should lead to much better visual performance.