This makes IPC GET_PIXELS use the new `wlc_pixels_read` call instead of
the deprecated `wlc_output_get_pixels`.
The old version worked by passing a callback function to wlc which would
grab the pixels and send them to the IPC client.
The new version works by maintaining a list of clients who have
requested the pixels of some output and then grap and send the pixels in
the output_post_render hook of the `wlc_interface`.
6f7cbf2eac