Implement OpenGL ES 2 support for WinRT QPA

If OpenGL ES 2 is available, the winrt platform plugin will use it
for the backing store implementation and provide a platform context.
Otherwise, it falls back to the DirectX blitter/page flipper.

Change-Id: I2f44d8d97dad5fe94984aaae883c6c76f1528006
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>