Function gl::ReadnPixels

pub unsafe fn ReadnPixels(
    x: GLint,
    y: GLint,
    width: GLsizei,
    height: GLsizei,
    format: GLenum,
    type_: GLenum,
    bufSize: GLsizei,
    data: *mut c_void
)

Fallbacks: ReadnPixelsARB, ReadnPixelsEXT, ReadnPixelsKHR