Fix / silence various new Rust 1.83 clippy warnings #4028

Job	Run time
build GTK binaries with gvsbuild	12s
build gtk-rs on Windows (cairo, true, --features png,pdf,svg,ps,use_glib,v1_16,freetype,script,wi...	1m 36s
build gtk-rs on Windows (gdk-pixbuf, true, --features v2_42)	1m 41s
build gtk-rs on Windows (gio, true, --features v2_80)	2m 10s
build gtk-rs on Windows (glib, true, --features v2_80)	3m 26s
build gtk-rs on Windows (glib-build-tools, false)	1m 29s
build gtk-rs on Windows (graphene, false)	1m 10s
build gtk-rs on Windows (pango, true, --features v1_52)	1m 33s
build gtk-rs on Windows (pangocairo, true)	2m 6s
build gtk-rs on Windows (examples, false, --bins --examples --all-features)	1m 40s
	17m 3s

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