Function read_back_rgba 

pub(crate) fn read_back_rgba(
    renderer: &mut GlesRenderer,
    target: &mut GlesTarget<'_>,
    width: i32,
    height: i32,
    dst: &mut [u8],
)

Reads target back as RGBA into dst. The read runs in a frame of the target: a dmabuf bound as a texture gets a framebuffer of its own on each bind, which only a frame makes current, so a bare read on a tick that drew nothing would read another framebuffer.