Add Rust API to grab the contents of a slint::Window into a slint::SharedImageBuffer #15547

Job	Run time
python_test (ubuntu-22.04)	25s
python_test (macos-12)	35s
python_test (windows-2022)	34s
cpp_test_driver (ubuntu-22.04)	22s
build_and_test (ubuntu-22.04, stable)	1s
cpp_test_driver (macos-12)	23s
cpp_cmake (ubuntu-22.04)	7s
build_and_test (ubuntu-22.04, 1.73)	1s
cpp_test_driver (windows-2022)	10s
mcu (pico-st7789, thumbv6m-none-eabi)	1s
cpp_cmake (macos-12)	1s
build_and_test (macos-12, stable)	1s
mcu (stm32h735g, thumbv7em-none-eabihf)	1s
cpp_cmake (windows-2022)	1s
build_and_test (windows-2022, stable)	1s
mcu_esp	1s
build_and_test (windows-2022, 1.73)	1s
build_and_test (windows-2022, beta, --exclude ffmpeg --exclude gstreamer-player)	1s
node_test (ubuntu-22.04)	1s
android	1s
node_test (macos-12)	1s
node_test (windows-2022)	1s
esp-idf-quick	1s
ffi_32bit_build	1s
fmt_test	1s
miri	1s
updater_test (0.3.0)	1s
uefi-demo	1s
docs / docs	1s
tree-sitter / tree-sitter-tests	1s
wasm / wasm	1s
wasm_demo / wasm_demo	1s
wasm / lint-slintpad	1s
wasm / build-slintpad	1s
cpp_package_test	1s
	3m 4s

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