[diagrams] path bounds and rendering multiple diagrams to a single document

[sam griffin]

Excellent!

Two questions I've been saving (Brent gets these twice, because I'm foolish):


First, the bounding box built for paths seems twice as big as it
should be (that is, the Bounds vector is twice as big). I had assumed
that the following code for a square would be a drop-in replacement
for box, but it is not.

polygon :: (BSpace b ~ P2, Renderable (Path P2) b) => [P2] -> Diagram b
polygon vlist = stroke . close $ pathFromVertices vlist

square :: (BSpace b ~ P2, Renderable (Path P2) b) => Diagram b
square = polygon [(-1,-1), (1,-1), (1,1), (-1,1)]


Second, if we want the Cairo Backend to render multiple diagrams into
a document, that will go beyond the bounds of what is defined for the
Backend class... should that be amended? Ideally, we could get around
that opening the desired document, rendering a single diagram to the
middle of it in whatever desired page, then saving it. Unfortunately,
the libraries we depend on probably don't support this.