Added null pointers for background image and eval positions

[weinman]

Updated the interface in pydiffvg_tensorflow/render_tensorflow.py to use all the arguments to the function render in diffvg.cpp. The placeholders (arguments background_image, d_background_image, eval_positions and num_eval_positions are simply NULL for now—0 (zero) in the latter case of an int, which defaults to evaluating over the entire raster domain), so that the default behavior results and the sample app in apps/single_circle_tf.py runs successfully. Fixes #7.

[BachiLi]

thanks!