Oculus: Oculus Rift DK2

This adds support for Oculus Rift DK2 virtual reality display.

## Supported Version

Version 0.5.0.1 beta is the ONLY SDK tested and supported for Windows and GNU/Linux.

## Dependencies

Oculus Rift SDK: SDK is available for Windows and GNU/Linux on Oculus Rift Developer website.

For Windows, you must first install the Oculus Runtime package and download the Oculus SDK. Then set an environment variable OCULUSSDK_DIR to the directory containing `LibOVR` (but do not include 'LibOVR' in the variable). For GNU/Linux, download the SDK package, extract it, run its configuration script FIRST, build the library, then install it. If the SDK is in a custom location, you must set an environment variable OCULUSSDK_DIR pointing to the install path. The default installed one will be picked if both exist.

Then, in CMake, enable BUILD_DEVICE_OCULUS.