Consecutive raster draws now merged

Elements of a vector output can be individually set to rasterized, using

the ``rasterized`` keyword, or `~.artist.Artist.set_rasterized()`. This can be useful to

reduce file sizes. For figures with multiple raster elements they are now

automatically merged into a smaller number of bitmaps where this will not

effect the visual output. For cases with many elements this can result in

significantly smaller file sizes.

To ensure this happens do not place vector elements between raster ones.