Function ReadnPixels 

pub unsafe fn ReadnPixels(
    x: i32,
    y: i32,
    width: i32,
    height: i32,
    format: u32,
    type_: u32,
    bufSize: i32,
    data: *mut c_void,
)

Fallbacks: ReadnPixelsARB, ReadnPixelsEXT, ReadnPixelsKHR