Fix incorrect offset of `PopupMenu` separator icons #13837

Job	Run time
📊 Static checks / Code style, file formatting, and docs	38s
🤖 Android / Template (target=template_release)	4m 21s
🍏 iOS / Template (target=template_release)	2m 47s
🍎 macOS / Editor (target=editor, tests=yes)	5m 7s
🐧 Linux / Editor w/ Mono (target=editor)	6m 20s
🍎 macOS / Template (target=template_release)	3m 5s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	19m 41s
🌐 Web / Template (target=template_release)	2m 18s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	8m 5s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	12m 32s
🐧 Linux / Template w/ Mono (target=template_release)	1m 6s
🏁 Windows / Editor (target=editor, tests=yes)	13m 25s
🏁 Windows / Template (target=template_release)	9m 36s
🐧 Linux / Minimal template (target=template_release, everything disabled)	1m 8s
🪲 Godot CPP / Build and test Godot CPP	10m 39s
	1h 40m 48s

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