Separate Latex and Photon rewriting src function

Latex and Photon was rewriting img src function was combined. I am separating this functions for now.
Changed position and documentation of imgNeedsSizeAtts

imgNeedsSizeAtts position is wrong along with its documentation. I've moved it below the constructor and changed the documentaion to something more clear.
Added getImgSrcWithZoom to avoid code duplication

There is some code duplication in the various src calculations functions. to avoid it, I've created getImgSrcWithZoom function to include the calculation and placement to be called from three functions.