The "future-dev" branch of MSOpenTech's ANGLE/WinRT repository (at
https://github.com/msopentech/angle) includes support for Windows Phone 8.1.
This change allows it to be used in conjunction with SDL's OpenGL functions.
since the window system doesn't do it for us like other platforms.
This prevents sticky keys and missed keys when going in and out
of focus, for example Alt would appear to stick if switching away
from an SDL app with Alt-Tab and had to be pressed again.
CR: Sam