Function read_back_rgba_result 

pub(crate) fn read_back_rgba_result(
    renderer: &mut GlesRenderer,
    target: &mut GlesTarget<'_>,
    width: i32,
    height: i32,
    dst: &mut [u8],
    check_error: bool,
) -> Result<(), String>

Screenshot reads also check GL errors before any pixels can be handed to the caller.