Add screenshot function to Viewer

• Take screenshots with F2, or a menu entry
  • Currently does not render UI due to GlfwApps architecture, might not be desirable anyway
  • Requires glow !35 to correctly output non-mirrored screenshots
• Adjust viewer scrolling speed based on scene scale
• Add "Quit" and "Quit All" option to viewer menu

camera/glow-extras/camera/controllers/TargetOrbitController.cc

@@ -18,6 +18,6 @@ void glow::camera::TargetOrbitController::update(float, const glow::input::Input
     auto const& scrollY = input.getScrollDelta().y;
     if (std::abs(scrollY) > 0.f)
     {
-        camera.handle.addTargetDistance(-scrollY * 0.1f);
+        camera.handle.addTargetDistance(-scrollY * mCameraScrollingSpeed);
     }
 }