Fix LightmapGI shading sometimes being unlit or black #11279

Job	Run time
📊 Static checks / Code style, file formatting, and docs	1m 14s
🍎 macOS / Editor (target=editor, tests=yes)	7m 34s
🍎 macOS / Template (target=template_release)	28m 43s
🤖 Android / Template (target=template_release)	5m 26s
🌐 Web / Template (target=template_release)	26m 16s
🏁 Windows / Editor (target=editor, tests=yes)	11m 18s
🐧 Linux / Editor w/ Mono (target=editor)	6m 7s
🏁 Windows / Template (target=template_release)	9m 29s
🐧 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)	18m 22s
🍏 iOS / Template (target=template_release)	25m 48s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	12m 31s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	34m 51s
🐧 Linux / Template w/ Mono (target=template_release)	2m 21s
🐧 Linux / Minimal template (target=template_release, everything disabled)	19m 6s
🪲 Godot CPP / Build and test Godot CPP	9m 50s
	3h 38m 56s

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