gltf_viewer: add --headless option.

This should only be used in combination with --batch. Users are
admonished if they try using it on its own.

libs/viewer/src/AutomationEngine.cpp

@@ -25,6 +25,7 @@
 
 #include <backend/PixelBufferDescriptor.h>
 
+#include <utils/Log.h>
 #include <utils/Path.h>
 
 #include <iomanip>
@@ -133,6 +134,9 @@ void AutomationEngine::tick(View* view, MaterialInstance* const* materials, size
         for (size_t i = 0; i < materialCount; i++) {
             applySettings(mSettings->material, materials[i]);
         }
+        if (mOptions.verbose) {
+            utils::slog.i << "Running test " << mCurrentTest << utils::io::endl;
+        }
     };
 
     if (!mIsRunning) {