title: GPUImage apply filter to a buffer of images
body: <p>In <code>GPUImage</code> there are some filters that works only for a stream of frames from a camera, for instance the low pass filter, or the high pass filter, but there are plenty of them. <br>
  I'm trying to create a buffer of UIImages that with a fixed timerate make possible to apply those filters also between just 2 images, and that for each pair of image produces a single filtered image. Something like that:<br>
  FirstImage+SecondImage-->FirstFilteredImage<br>
  SecondImage+ThirdImage-->SecondFilteredImage<br>
  I've found that filters that works with frames use a <code>GPUImageBuffer</code>, that is a subclass of <code>GPUImageFilter</code> (most probably just to inherit some methods and protocols) that loads a passthrough fragment shader. From what I understood this is a buffer that keeps incoming frames but already "texturized", textures are generated by binding the texture in the current context. <br>
  I've found also a <code>-conserveMemoryForNextFrame</code> that sounds good for what I want to achieve, but I didn't get how is working.<br>
  Is it possible to do that? in which method images are converted in texture?</p>