Tracking of depth of raster draws has moved from .backend_mixed.MixedModeRenderer.start_rasterizing and .backend_mixed.MixedModeRenderer.stop_rasterizing into .artist.allow_rasterization. This means the start and stop functions are only called when the rasterization actually needs to be started and stopped.

The output of vector backends will change in the case that rasterized elements are merged. This should not change the appearance of outputs.

The renders in 3rd party backends are now expected to have self._raster_depth and self._rasterizing initialized to 0 and False respectively.