Meta: Disable "Zoom To Fit" when using the gtk display backend

This option causes the window to not resize automatically on guest
display resolution changes.
QEMU has this option enabled by default when using virtio-gpu.

Meta/run.py

@@ -621,6 +621,10 @@ def set_up_screens(config: Configuration):
         else:
             config.display_backend = "gtk,gl=off"
 
+    # Disable "Zoom To Fit" because it breaks absolute mouse input and looks ugly.
+    if config.display_backend.startswith("gtk"):
+        config.display_backend += ",zoom-to-fit=off"
+
 
 def set_up_display_device(config: Configuration):
     config.enable_gl = environ.get("SERENITY_GL") == "1"