Fix wrong indexing when generating dummy tangents in GLTF import #235

Job	Run time
📊 Static checks / Code style, file formatting, and docs	4m 30s
🍎 macOS / Editor (target=editor, tests=yes)	21m 43s
🐧 Linux / Editor w/ Mono (target=editor)	39m 10s
🍎 macOS / Template (target=template_release)	22m 21s
🤖 Android / Template (target=template_release)	30m 26s
🐧 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)	40m 27s
🍏 iOS / Template (target=template_release)	22m 58s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	38m 51s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	28m 56s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	22m 57s
🐧 Linux / Template w/ Mono (target=template_release)	31m 5s
🏁 Windows / Editor (target=editor, tests=yes)	45m 12s
🌐 Web / Template w/o threads (target=template_release, threads=no)	22m 24s
🐧 Linux / Minimal template (target=template_release, everything disabled)	19m 20s
🏁 Windows / Template (target=template_release)	31m 37s
🪲 Godot CPP / Build and test Godot CPP	6m 54s
	7h 8m 51s

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