Transmutes from non repr(C) to repr(C) structs about rust-azure

[Ms2ger]

This was mostly fixed by #251, but not entirely. The following are left:

    pub fn create_gradient_stops(&self,
                                 gradient_stops: &[GradientStop],
                                 extend_mode: ExtendMode)
                                 -> GradientStops {
        unsafe {
            GradientStops::new(AzDrawTargetCreateGradientStops(
                    self.azure_draw_target,
                    mem::transmute::<_,*const AzGradientStop>(gradient_stops.as_ptr()),
                    gradient_stops.len() as u32,
                    extend_mode.as_azure_extend_mode()))
        }
    }

    pub fn set_transform(&self, matrix: &Matrix2D<AzFloat>) {
        unsafe {
            AzDrawTargetSetTransform(self.azure_draw_target, mem::transmute(matrix));
        }
    }

CC @nox

[nox]

@Ms2ger The first one is ok because GradientStop is actually repr(C). The second one though, clearly an oversight on my part.