Fix GLTF exporting invalid meshes and attempting to export gizmo meshes #21603

Job	Run time
📊 Static checks / Code style, file formatting, and docs	43s
🍏 iOS / Template (target=template_release)	36m 34s
🐧 Linux / Editor w/ Mono (target=editor)	35m 17s
🐧 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)	35m 25s
🤖 Android / Template (target=template_release)	29m 43s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	26m 35s
🍎 macOS / Editor (target=editor, tests=yes)	48m 10s
🍎 macOS / Template (target=template_release)	35m 38s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	27m 57s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	21m 15s
🐧 Linux / Template w/ Mono (target=template_release)	23m 28s
🌐 Web / Template w/o threads (target=template_release, threads=no)	21m 16s
🐧 Linux / Minimal template (target=template_release, everything disabled)	14m 23s
🏁 Windows / Editor (target=editor, tests=yes)	41m 19s
🏁 Windows / Template (target=template_release)	29m 56s
🪲 Godot CPP / Build and test Godot CPP	6m 51s
	7h 14m 30s

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