Possible Incompleteness

[YichiZhang0613]

In glium/src/framebuffer/mod.rs, I think the code should check samples != Some(0) and layers != Some(0) as comments mentioned before use them to avoid possible panic.

 /// # Panic
    ///
    /// Panics if `layers` or `samples` is equal to `Some(0)`.
    ///
    #[inline]
    pub fn new<F: ?Sized>(facade: &F, width: u32, height: u32, layers: Option<u32>,
                  samples: Option<u32>, fixed_samples: bool)
                  -> Result<EmptyFrameBuffer, ValidationError> where F: Facade
    {
        let context = facade.get_context();

        let attachments = fbo::FramebufferAttachments::Empty {
            width,
            height,
            layers,
            samples,
            fixed_samples,
        };