vg: consider migrating vg.Length and vg.Point to x/image/math/fixed

The package [`golang.org/x/image/math/fixed`](https://godoc.org/golang.org/x/image/math/fixed) defines fixed point integer types such as `fixed.Int26_6` and `fixed.Int52_12` (and corresponding `Point` and `Rectangle` struct types).

migrating `vg.Length` from:

``` go
package vg
type Length float64
```

to:

``` go
package vg
import "golang.org/x/image/math/fixed"
type Length fixed.Int26_6
```

would (_perhaps_) ease interoperabiliy with all the other 2D packages.