Introduced outputTextureOptions for GPUImageOutput.

This allows to modify the output texture settings of a filter to, for example, allow float-typed output textures.
Further, one can now optimize the memory usage of filters which do not need four color components, e.g., by using the GL_LUMINANCE format instead of GL_RGBA.

I also tried to introduce the new options in the existing filters. However, I'm not quite sure if I covered them all.
For some places—especially when reading back values from the texture—it did not seam appropriate to change the texture options. I added some assertions to ensure the options are still the defaults.