Support SVG overlay generation

[bwsw]

Allow users to asynchronously and dynamically generate SVG graphics for their video frames.

#[derive(Debug, Clone)]
enum ImageResult {
  NotReady,
  NotAvailable(String),
  Image {
    buffer: Arc<Vec<u8>>,
    cached: bool
  }
}

impl LayoutGenerator {
  pub fn new(cache_time: Duration, cache_size: usize) -> Self;
  pub fn update(id: &str, svg_doc: &str, sync_wait: bool) -> anyhow::Result<&ImageResult>;
  pub fn get(id: &str) -> anyhow::Result<ImageResult>;
}