This extracts non-ANGLE specific code to d3d11_helpers.c, which is
modeled after egl_helpers.c. Currently the only consumer is
context_angle.c, but in future this may allow the D3D11 device and
swapchain creation logic to be reused in other backends.
Also includes small improvements to D3D11 device creation. It is now
possible to create feature level 11_1 devices (though ANGLE does not
support these,) and BGRA swapchains, which might be slightly more
efficient than ARGB, since its the same format used by the compositor.
James Ross-Gowan