fix glyph string analysis methods that don't need &mut #3054

Job	Run time
build GTK binaries with gvsbuild	27s
build gtk-rs on Windows (cairo, true, --features png,pdf,svg,ps,use_glib,v1_16,freetype,script,wi...	2m 46s
build gtk-rs on Windows (gdk-pixbuf, true, --features v2_42)	2m 26s
build gtk-rs on Windows (gio, true, --features v2_74)	3m 57s
build gtk-rs on Windows (glib, true, --features v2_74)	4m 36s
build gtk-rs on Windows (glib-build-tools, false)	1m 29s
build gtk-rs on Windows (graphene, false)	1m 50s
build gtk-rs on Windows (pango, true, --features v1_50)	2m 39s
build gtk-rs on Windows (pangocairo, true)	3m 20s
build gtk-rs on Windows (examples, false, --bins --examples --all-features)	3m 35s
	27m 5s

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