Function compile_to_stencil_offthread 

pub fn compile_to_stencil_offthread<F>(
    options: *const ReadOnlyCompileOptions,
    source: Arc<String>,
    callback: F,
) -> OffThreadToken
where
    F: FnOnce(CompilationResult) -> Option<CompilationResult> + Send + 'static,

Creates a new thread and starts compilation there

Callback receives stencil that can either consumed or returned