Rasterize 2D Landmarks Method

[patricksnape]

This method allows rasterizing of landmark groups onto an image.
Multiple landmark groups can be rasterized onto the same
image for efficiency. Currently two backends are provided, pillow
and matplotlib. The matplotlib backend is far more feature
complete, but slow! The pillow backend is much simpler but
an order of magnitude faster than the matplotlib backend.

@nontas check this out - it should be something you will like 😄

[nontas]

That looks great @patricksnape !!
My only comment is that since this PR replaces plt.scatter with plt.plot for rendering points (and also correctly changes marker_size default value from 20 to 5), we should also change the marker_size documentation in our viewable classes (e.g. Image, PointCloud, PointGraph etc.). marker_size is no longer The size of the markers in points^2. but The size of the markers in points.

[nontas]

+1

[patricksnape]

@jabooth Want to check this too?

[jabooth]

Works well! Only comment is that I think a fairly likely that users may wish to render all landmarks out, and atm that would require something like:

img.rasterize_landmarks_2d(group=img.landmarks.keys())

Saying that, in such cases you would need to know what order the keys are in to choose different colours etc for the landmarks. The API chosen here is consistent with what we have elsewhere in menpo, so let's get this in.

+1