SDLGPU: Remove manual fence synchronization #796

Job	Run time
Rocky Linux	1m 59s
Debian Linux PPC	1m 52s
FreeBSD	1m 11s
Rocky Linux SDL3	1m 29s
Rocky Linux MinGW	57s
macOS (x86_64)	27s
Windows (MSVC)	1m 11s
iOS/tvOS SDL3 (iOS, iphoneos, arm64)	40s
Rocky Linux MinGW SDL3	1m 0s
iOS/tvOS SDL3 (iOS, iphonesimulator, x86_64;arm64)	46s
iOS/tvOS SDL3 (tvOS, appletvos, arm64)	44s
iOS/tvOS SDL3 (tvOS, appletvsimulator, x86_64;arm64)	44s
Debian Linux PPC SDL3	56s
macOS (Universal) SDL3	55s
	14m 51s

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