Make SDL_RegisterEvents() thread-safe #4620

Job	Run time
Create test plan	9s
level1 / Haiku	1m 39s
level2 / Windows (MSVC, clang-cl x64)	6m 26s
level2 / Sony PlayStation Portable	3m 57s
level2 / LoongArch64	1m 59s
level2 / Sony PlayStation 2	5m 2s
level2 / MacOS (GNU prefix)	4m 5s
level2 / MacOS (Framework) (arm64)	2m 32s
level2 / Emscripten	8m 32s
level2 / Windows (msys2, mingw64)	4m 33s
level2 / MacOS (Framework) (x64)	5m 50s
level2 / NetBSD	3m 54s
level2 / PowerPC	2m 2s
level2 / Windows (MSVC, ARM64)	4m 23s
level2 / Sony PlayStation Vita (GLES w/ PVR_PSP2)	2m 52s
level2 / iOS (CMake & xcode)	2m 4s
level2 / Windows (msys2, clang64)	4m 7s
level2 / Windows (MSVC, ARM)	4m 29s
level2 / Windows (MSVC, x86)	6m 25s
level2 / Android (CMake)	2m 39s
level2 / Android (CMake, lean)	2m 1s
level2 / PowerPC64	2m 0s
level2 / Ubuntu 22.04 (Intel Compiler)	3m 41s
level2 / Android (Gradle)	2m 45s
level2 / Nintendo 3DS	1m 57s
level2 / Windows (MSVC, clang-cl x86)	6m 6s
level2 / Windows (msys2, mingw32)	5m 25s
level2 / Ubuntu 24.04 (ARM64)	5m 43s
level2 / Android (Android.mk)	1m 26s
level2 / Windows (MSVC, x64)	5m 14s
level2 / Ubuntu 22.04 (Intel oneAPI)	4m 29s
level2 / FreeBSD	2m 48s
level2 / GDK (MSVC, x64)	2m 55s
level2 / Windows (msys2, ucrt64)	4m 40s
level2 / Ubuntu 22.04	2m 31s
level2 / RISC OS	1m 29s
level2 / Steam Linux Runtime (Sniper)	2m 46s
level2 / tvOS (CMake & xcode)	2m 36s
level2 / Sony PlayStation Vita (GLES w/ pib)	2m 47s
	2h 20m 58s

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