Fix memory corruption when loading encoded embedded images in C++

[tronical]

The ImageCacheKey enum has a gap when not targeting wasm:

pub enum ImageCacheKey {
    /// This variant indicates that no image cache key can be created for the image.
    /// For example this is the case for programmatically created images.
    Invalid,
    /// The image is identified by its path on the file system.
    Path(SharedString),
    /// The image is identified by a URL.
    #[cfg(target_arch = "wasm32")]
    URL(SharedString),
    /// The image is identified by the static address of its encoded data.
    EmbeddedData(usize),
}

In the C++ generated header, that cfg was not mapped, and thus the URL variant was always there, while in a regular slint-cpp build it's not. Consequently tag value 2 in Rust was used to represent the EmbeddedData variant, while in the C++ generated code that become variant 3 and 2 was interpreted as a URL. So the receiving code in C++ tried to interpret the cache key as URL variant, while Rust created it as EmbeddedData.