ensure textures are always destroyed

[mBornand]

When the previous wallpaper was replaced,
nothing was cleaning up the texture.

This is achieved by moving the destruction of the texture to the wallpaper code. This way we know it will be destroyed each time.

Without this fix I have the same or a similar issue as #81.

[danyspin97]

Thanks for the PR @mBornand! I am currently in vacation, I'll review the PR first thing next week :)

[danyspin97]

I finally see the issue:

    #[inline]
    pub fn transition_finished(&mut self) {
        // By binding transparent pixel into the old wallpaper, we can delete the texture,
        // freeing space from the GPU
        unsafe {
            // The previous wallpaper is always binding in TEXTURE0
            self.gl.ActiveTexture(gl::TEXTURE0);
            self.gl
                .BindTexture(gl::TEXTURE_2D, self.transparent_texture);
            self.prev_wallpaper.take();
        }
    }

When I removed the previous wallpaper by calling Option::take(), the texture was not deleted. With this PR the issue is fixed, because now the texture is properly deleted. Thank you so much @mBornand !