Add Capture Screenshot API

[gfrolov]

This is a feature request to add API to capture screenshot in DirectMode and/or ExtendedMode. This would work similar to OpenVR's IVRScreenshots::TakeStereoScreenshot by taking the submitted texture and write two images (mirror image (not split) and VR side by side image). There is also a wiki page for screenshot API overview.
Proposed API :
osvrRenderManagerTakeScreenshot(OSVR_RenderManager renderManager, const char *destinationPath, const char *previewFileName, const char *vrFileName)
destinationPath is an optional parameter where we can use a default path instead (current binary location for example).