OpenVR SDK 1.0.4:

General:
* Further refinement to Vulkan support. Note: the interface is still subject to change.

IVRSystem (v14):
* ComputeDistortion signature change. Previous version used nan as error signaling. New version returns explicit success.
* GetHiddenAreaMesh takes optional EHiddenAreaMeshType argument to request Standard, Inverse, or LineLoop mesh.

IVRCompositor (v18):
* New submit error, VRCompositorError_AlreadySubmitted (if you submit the same eye twice)
* GetFrameTimings signature change
* Exposed GetCurrentFadeColor, GetCurrentGridAlpha

IVROverlay (v13):
* Added SetOverlayIntersectionMask

IServerDriverHost:
* Added PollNextEvent

IClientDriverHost:
* Removed GetSettings
* Added GetGenericInterface

ClientTrackedDeviceProvider (v5):
* GetHiddenAreaMesh takes additional argument, EHiddenAreaMeshType

Pull Requests integrated:
#48 Format README in Markdown
#58 correct detection of GCC
#68 helloworldoverlay: Specify c++11 to fix gcc errors
#76 Remove duplicate, unneeded glBindBuffer call.
#135 Invert m_mat4HMDPose after assignment copying.
#191 Document additional methods
#278 include stdbool.h on non windows platform
#306 Multi-platform CMake support.

[git-p4: depot-paths = "//vr/steamvr/sdk_release/": change = 3697868]