Fix FogMaterial memory leak #15379

Job	Run time
📊 Static checks / Code style, file formatting, and docs	1m 10s
🌐 Web / Template (target=template_release)	25m 57s
🍎 macOS / Editor (target=editor, tests=yes)	35m 36s
🍎 macOS / Template (target=template_release)	25m 8s
🤖 Android / Template (target=template_release)	26m 45s
🍏 iOS / Template (target=template_release)	23m 6s
🐧 Linux / Editor w/ Mono (target=editor)	1h 0m 20s
🐧 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)	10m 55s
🏁 Windows / Editor (target=editor, tests=yes)	39m 59s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	29m 15s
🏁 Windows / Template (target=template_release)	49m 12s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	26m 35s
🐧 Linux / Template w/ Mono (target=template_release)	23m 0s
🐧 Linux / Minimal template (target=template_release, everything disabled)	20m 48s
🪲 Godot CPP / Build and test Godot CPP	10m 31s
	6h 48m 17s

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