Fix usage of index offsets in RenderingDevice #19307

Job	Run time
📊 Static checks / Code style, file formatting, and docs	42s
🍎 macOS / Editor (target=editor, tests=yes)	25m 24s
🍎 macOS / Template (target=template_release)	23m 10s
🤖 Android / Template (target=template_release)	17m 9s
🍏 iOS / Template (target=template_release)	10m 44s
🐧 Linux / Editor w/ Mono (target=editor)	25m 21s
🌐 Web / Template (target=template_release)	11m 48s
🏁 Windows / Editor (target=editor, tests=yes)	29m 27s
🐧 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)	25m 51s
🏁 Windows / Template (target=template_release)	18m 41s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	24m 30s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	22m 27s
🐧 Linux / Template w/ Mono (target=template_release)	14m 40s
🐧 Linux / Minimal template (target=template_release, everything disabled)	11m 27s
🪲 Godot CPP / Build and test Godot CPP	6m 44s
	4h 28m 5s

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