Add Rust API to grab the contents of a slint::Window into a slint::SharedImageBuffer #15558

Job	Run time
node_test (ubuntu-22.04)	3m 39s
cpp_test_driver (ubuntu-22.04)	6m 39s
node_test (macos-12)	3m 39s
cpp_test_driver (macos-12)	5m 33s
node_test (windows-2022)	4m 29s
cpp_cmake (ubuntu-22.04)	6m 34s
build_and_test (ubuntu-22.04, stable)	6m 28s
cpp_test_driver (windows-2022)	6m 17s
cpp_cmake (macos-12)	6m 34s
build_and_test (ubuntu-22.04, 1.73)	6m 32s
cpp_cmake (windows-2022)	6m 29s
build_and_test (macos-12, stable)	6m 36s
ffi_32bit_build	1m 7s
build_and_test (windows-2022, stable)	6m 14s
mcu (pico-st7789, thumbv6m-none-eabi)	39s
build_and_test (windows-2022, 1.73)	6m 23s
mcu (stm32h735g, thumbv7em-none-eabihf)	1m 15s
mcu_esp	2m 36s
python_test (ubuntu-22.04)	3m 43s
build_and_test (windows-2022, beta, --exclude ffmpeg --exclude gstreamer-player)	6m 23s
python_test (macos-12)	1m 42s
uefi-demo	35s
python_test (windows-2022)	1m 59s
android	2m 8s
fmt_test	2m 46s
miri	43s
esp-idf-quick	2m 49s
updater_test (0.3.0)	1m 44s
docs / docs	2m 5s
wasm / wasm	2m 0s
tree-sitter / tree-sitter-tests	12s
wasm_demo / wasm_demo	1m 3s
cpp_package_test	1s
wasm / lint-slintpad	1s
wasm / build-slintpad	1s
	1h 57m 38s

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