Win32: Add a command-line argument to pick the GPU backend.

It makes sense to add it since some users can't even open PPSSPP in OpenGL mode, so they can't get an initial ini file, meaning they need to copy and paste a fresh ini file into a blank ppsspp.ini, which is extremely unintuitive, so let this serve as a workaround for it until we add backend fallbacks. After that, it could be useful for debugging or something, perhaps..

Example syntax:
ppssppwindows.exe --gfx=ogl
ppssppwindows.exe --gfx=d3d

The former will launch PPSSPP with the OpenGL backend, whilst the latter will launch it with the Direct3D backend.