Change `Point` to be a Double

[karwa]

Both AGG and CoreGraphics draw lines, curves, etc natively with double precision. It would be nice if our Points were also Doubles (rather than Floats). I doubt it makes a perceptible difference to the output (maybe at retina scales where the images are drawn 2-3x their normal size?), but it would reduce the number of conversions we need to do and just generally make things cleaner.