Fix deadlock possibility in threaded load of materials #1051

Job	Run time
📊 Static checks / Code style, file formatting, and docs	19s
🍏 iOS / Template (target=template_release)	1m 31s
🤖 Android / Editor (target=editor)	3m 34s
🍎 macOS / Editor (target=editor, tests=yes)	41s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	2m 58s
🐧 Linux / Editor w/ Mono (target=editor)	3m 15s
🍎 macOS / Template (target=template_release)	32s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	43s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	2m 59s
🐧 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 35s
🌐 Web / Template w/o threads (target=template_release, threads=no)	3m 41s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	6m 52s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	6m 15s
🏁 Windows / Editor (target=editor, tests=yes)	10m 56s
🐧 Linux / Template w/ Mono (target=template_release)	1m 46s
🏁 Windows / Template (target=template_release)	9m 57s
🐧 Linux / Minimal template (target=template_release, everything disabled)	1m 43s
🪲 Godot CPP / Build and test Godot CPP	7m 52s
	1h 16m 9s

Provide feedback