Function read_back_rgba_async 

pub(crate) fn read_back_rgba_async(
    renderer: &mut GlesRenderer,
    readbacks: &mut ScreenshotReadbacks,
    target: &mut GlesTarget<'_>,
    width: i32,
    height: i32,
) -> Option<Result<GpuReadback, String>>

Start reading the bound target back into a pixel-pack buffer and return the readback the caller waits for, or None where the buffers are unavailable.