Fix Proxy interface

Update/cleanup of the transform and get_image and set_image interface, with the goal of being able to run the Python examples again.

Fixed the merge conflicts with the master branch, which apparently had been updated concurrently.

Some of the changes applied there have been undone, most notably the additional arguments in the set_image method and the member variables in the CPU proxy.

The code now has init_wtiles and flush_wtiles, to keep the functionality separated.