gtk4: scaling regression in 0.12

To reproduce:

• Run this test app: https://gitlab.freedesktop.org/gdesmott/test-gst/-/blob/gtk4/src/main.rs?ref_type=heads
• Unfullscreen it by hitting "Escape"

Here is how it looks with 0.12:

and this is with 0.11 (checkout HEAD^ in this repo):

The regression has been introduced by !1309 (3423d05f)