SDL: Always set clear_depth/clear_stencil #843

Job	Run time
Debian Linux PPC	1m 22s
Fedora Linux MinGW	1m 9s
iOS/tvOS (iOS, iphoneos, arm64)	1m 3s
Steam Linux Runtime	1m 11s
iOS/tvOS (iOS, iphonesimulator, x86_64;arm64)	1m 20s
macOS (Universal)	1m 15s
Steam Linux Runtime (SDL2)	1m 21s
iOS/tvOS (tvOS, appletvos, arm64)	52s
iOS/tvOS (tvOS, appletvsimulator, x86_64;arm64)	1m 9s
Windows MSVC	1m 43s
	12m 25s

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