Improve viewer screenshots, add camera orientation option

• Added glow::viewer::camera_orientation viewer config object, taking in azimuth, altitude and optional distance to center
• Added custom resolution option to screenshot tool
• Background grid hidden in screenshots
• Added Info tab to viewer menu display camera info (same format as taken by camera_orientation)
• Added "Reset Camera" action to viewer menu, hotkey F3 (resets to position dictated by scene, compatible with interactive mode)