make upload command buffer operations thread-safe #747

Job	Run time
Rocky Linux MinGW	1m 9s
FreeBSD	1m 25s
Rocky Linux	1m 45s
iOS/tvOS SDL3 (iOS, iphoneos, arm64)	45s
Rocky Linux MinGW SDL3	55s
Debian Linux PPC	1m 52s
iOS/tvOS SDL3 (iOS, iphonesimulator, x86_64;arm64)	57s
Debian Linux PPC SDL3	1m 7s
iOS/tvOS SDL3 (tvOS, appletvos, arm64)	1m 1s
Rocky Linux SDL3	1m 29s
iOS/tvOS SDL3 (tvOS, appletvsimulator, x86_64;arm64)	1m 4s
macOS (x86_64)	38s
macOS (Universal) SDL3	1m 0s
Windows (MSVC)	1m 25s
	16m 32s

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