The base mixin for the template tags, PipelineMixin, performed the bulk
of its rendering logic in the render_compressed() method. It handled
both rendering output files (if PIPELINE_ENABLED was True), or source
files (if False). Due to the complexity of this method, it was difficult
for a subclass (which may need to perform more specialized logic) to
override this method without copying the code.

This splits out that function into two new utility methods,
render_compressed_sources() and render_compressed_output(). These take
the same arguments as render_compressed(), and are called by that method
as well. This allows a subclass to more easily replace the logic for
determining which to call, or to replace the logic for either of these
renders.