Windows: cache custom cursor

[daxpedda]

We might want to let CustomCursorBuilder::build() be able to return an error, but that's a discussion for the future.
For now, the behavior doesn't actually change, because before this PR we didn't return an error from Window::set_custom_cursor() as well.

[madsmtm]

Seeing this, I think the API should actually be something like:

impl CustomCursor {
    pub fn from_rgba(
        rgba: impl Into<Vec<u8>>,
        width: u16,
        height: u16,
        hotspot_x: u16,
        hotspot_y: u16,
    ) -> CustomCursorBuilder;
}

impl CustomCursorBuilder {
    pub fn build<T>(self, window_target: &EventLoopWindowTarget<T>) -> Result<CustomCursor, BadImage>;
}

But that can be done later.

[daxpedda]

Agreed, will do that in a follow-up.