CustomMarker add_path and get_compiled_path likely to fail on some backends

This issue could alternatively be "Compiled Path API inconsistent between backends".

The issue here is that both of these methods on `CustomMarker` try to provide a copy of the custom marker's path scaled appropriately for the size of the marker. This relies on a call to `CompiledPath.scale_ctm` which is not available on all backends (eg. QPainter backend, celiagg backend) and in some cases (eg. QPainter) there doesn't look to be an easy way to provide that support.

As a result the call will fail.

In some cases `add_path` may still work in use because it is being used by passing in a GC instead of a path.