vg: drop DPI from Canvas
Created by: kortschak
The DPI value is only ever used internally by types to determine the number of dots for a given Length, the alternative is to give the Dots method the DPI value rather than the Canvas.
The DPI method is retained on types that need to know their resolution, although this could be made unexported.
@eaburns This is exploratory, so I'm not fussed if you reject it, but it does drop one method from the vg.Canvas set and it doesn't really look like it impacts in a negative way to my mind. Please take a look.