Added support for choosing the rendering gpu using wgl_nv_affinity extension.

This patch allows users to choose which GPU the opengl rendering commands are sent to using the wgl_nv_affinity extension. The gpu is specified through a new 'GLFW_AFFINITY_GPU' hint. If the extension is unsupported on the current hardware, the hint is just ignored. If the hint is not set, a normal device context is created as usual.
For systems with multiple GPUs, using multiple windows this can greatly speed up rendering efficiency.

It also adds a new Native API call to get the device context.