Submodule bump & build update

* Update README to reflect new libjson-c requirements
* Update brushlib submodule to latest HEAD
  - now builds into itself, so update LIBPATH & rm previous files
test_compositeops.py: update to reflect moved strs
tests+doc: remove deprecated usages

Remove the old stroke_to() - either go through the controller's current
mode now, and let that handle stroke recording and splitting, or go through
the model's current layer.

Update tests accordingly, so that they're closer to what now happens in the
app. For test_memory_leak.py, not using the deprecated version also
cures a detected (but spurious) leak.