draw2d upstream change

Created by: brettowe

vgimg is broken currently without the below change

--- github.com/gonum/plot/vg/vgimg/vgimg.go.old 2015-08-15 13:49:34.049328696 -0700
+++ github.com/gonum/plot/vg/vgimg/vgimg.go 2015-08-15 13:49:37.186032505 -0700
@@ -18,6 +18,7 @@
    "io"

    "github.com/llgcode/draw2d"
+   "github.com/llgcode/draw2d/draw2dimg"
    "golang.org/x/image/tiff"

    "github.com/gonum/plot/vg"
@@ -92,7 +93,7 @@
    }
    if c.gc == nil {
        h := float64(c.img.Bounds().Max.Y - c.img.Bounds().Min.Y)
-       c.gc = draw2d.NewGraphicContext(c.img)
+       c.gc = draw2dimg.NewGraphicContext(c.img)
        c.gc.SetDPI(c.dpi)
        c.gc.Scale(1, -1)
        c.gc.Translate(0, -h)