SDLGPU: Remove manual fence synchronization #760

Job	Run time
Rocky Linux	1m 59s
iOS/tvOS SDL3 (iOS, iphoneos, arm64)	44s
FreeBSD	1m 17s
iOS/tvOS SDL3 (iOS, iphonesimulator, x86_64;arm64)	48s
iOS/tvOS SDL3 (tvOS, appletvos, arm64)	42s
Rocky Linux MinGW	1m 9s
Rocky Linux MinGW SDL3	1m 5s
iOS/tvOS SDL3 (tvOS, appletvsimulator, x86_64;arm64)	40s
Debian Linux PPC	1m 48s
Debian Linux PPC SDL3	1m 6s
Rocky Linux SDL3	1m 27s
macOS (x86_64)	36s
macOS (Universal) SDL3	1m 0s
Windows (MSVC)	43s
	15m 4s

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