make upload command buffer operations thread-safe #740

Job	Run time
Rocky Linux MinGW	1m 14s
Rocky Linux	1m 47s
iOS/tvOS SDL3 (iOS, iphoneos, arm64)	50s
FreeBSD	1m 15s
iOS/tvOS SDL3 (iOS, iphonesimulator, x86_64;arm64)	53s
Rocky Linux MinGW SDL3	55s
Debian Linux PPC	1m 48s
iOS/tvOS SDL3 (tvOS, appletvos, arm64)	41s
iOS/tvOS SDL3 (tvOS, appletvsimulator, x86_64;arm64)	50s
Debian Linux PPC SDL3	1m 2s
Rocky Linux SDL3	1m 30s
macOS (x86_64)	38s
macOS (Universal) SDL3	57s
Windows (MSVC)	1m 17s
	15m 37s

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