GTK+ windows don't render when larger than 478x478

PPA containing a more recent GTK+: https://launchpad.net/~attente/+archive/ubuntu/gtk-mir

Under U8, GTK+ windows don't render when the width and height are both >= 479.

If the app starts at a size larger than that, the window will initially appear entirely black.

Once resized < 479 in either dimension, the window will render properly.

After that, when resizing larger than 478x478, the window only upscales the most recent framebuffer contents.

I haven't 100% confirmed that this is not GTK+. However:

- This bug does not occur when tested with miral-shell
- GTK+ is receiving configure events for surface resizes
- GTK+ is re-creating cairo surfaces that match the larger size
- GTK+ is still calling mir_buffer_stream_swap_buffers_sync at larger sizes
- I don't think this specific 478/479 number is coming from GTK+, the initial/min/max sizes aren't that close to it