MakePicture methods always create new TargetPicture

[duysqubix]

Look at batch.go#L82 and canvas.go#L73
All MakePicture methods return new TargetPicture instances.
Also every Sprite caches a TargetPicture.
So, does that means, for 10k Sprites created from a single Picture, pixel creates 10k TargetPictures?
That sounds very inefficient!

Original issue: faiface/pixel#178