Add support for HiDPI modes

When HiDPI is enabled, the size of the OpenGL surface (specified in
pixels) may no longer correspond to the size of the window (specified
in points). Width, Height, ClientSize and ClientRectangle return the
size of the OpenGL surface in device-dependent pixel coordinates
(origin: top-left pixel of the surface). Bounds and Size return the
size of the window in device-independent point coordinates (origin:
desktop).
Initial implementation of IJoystickDriver API