Allow preloading multiple background images to GPU for fast switching #12839

Job	Run time
Bundle test (macos-latest)	3m 56s
Linux (python=3.10 cc=gcc sanitize=0)	1m 55s
Linux package	3m 5s
Test ./dev.sh and benchmark	1m 35s
Linux (python=3.11 cc=gcc sanitize=1)	1m 56s
Bundle test (ubuntu-latest)	2m 2s
Linux (python=3.12 cc=clang sanitize=1)	2m 28s
macOS Brew	3m 45s
	20m 42s

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