Add viewer::headless_screenshot config

GLOW_VIEWER_CONFIG(glow::viewer::headless_screenshot(res, accumCount, filename));

Viewer will not open a window but instead render the specified screenshot and close immediately. (Uses global settings, cannot be nested or used within interactive viewers)

Not truly headless, a window is briefly created, not tested without X Server running. Some additional refactoring would be required for a real headless screenshot as GlfwApp is currently tightly coupled with the Viewer.