Poisson blending

I added a class GPUImagePoissonBlendFilter for iterative Poisson image blending.  The basic idea of Poisson blending is to blend the colors of one image with the gradients of another.  It's often used for seamlessly cutting and pasting objects (via their gradients) into a scene.

This also required adding a class GPUImageTwoInputCrossTextureSamplingFilter, which computes the value of a pixel based on its 4-neighborhood in two input images.