make upload command buffer operations thread-safe #741

Job	Run time
Rocky Linux MinGW	1m 12s
FreeBSD	1m 15s
iOS/tvOS SDL3 (iOS, iphoneos, arm64)	57s
Rocky Linux	1m 56s
iOS/tvOS SDL3 (iOS, iphonesimulator, x86_64;arm64)	1m 7s
Rocky Linux MinGW SDL3	42s
iOS/tvOS SDL3 (tvOS, appletvos, arm64)	52s
Debian Linux PPC	1m 48s
Debian Linux PPC SDL3	46s
iOS/tvOS SDL3 (tvOS, appletvsimulator, x86_64;arm64)	1m 0s
Rocky Linux SDL3	1m 4s
macOS (x86_64)	40s
Windows (MSVC)	1m 31s
macOS (Universal) SDL3	1m 15s
	16m 5s

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