Implement SysTexture for SDL_GPU backend #805

Job	Run time
Rocky Linux MinGW	52s
Rocky Linux	1m 43s
FreeBSD	36s
iOS/tvOS SDL3 (iOS, iphoneos, arm64)	51s
iOS/tvOS SDL3 (iOS, iphonesimulator, x86_64;arm64)	1m 17s
Rocky Linux MinGW SDL3	1m 24s
Debian Linux PPC	1m 42s
iOS/tvOS SDL3 (tvOS, appletvos, arm64)	44s
Debian Linux PPC SDL3	55s
iOS/tvOS SDL3 (tvOS, appletvsimulator, x86_64;arm64)	1m 2s
Rocky Linux SDL3	1m 32s
macOS (x86_64)	28s
macOS (Universal) SDL3	1m 4s
Windows (MSVC)	1m 1s
	15m 11s

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