Win32: On Optimus systems, use the NVidia GPU, not the IGP.

See https://github.com/dolphin-emu/dolphin/commit/b3ed3bdb910fe77dc3b1e05478258f693b5d943d and http://developer.download.nvidia.com/devzone/devcenter/gamegraphics/files/OptimusRenderingPolicies.pdf. Apparently we're not using the NVidia GPU on Optimus-based systems, so this should help, according to the PDF.