vg/draw: add a FillStyle

it should be possible to further customize the way one defines a "fill color", by _e.g._ adding a `vg/draw.FillStyle` that would allow to define filling patterns like so:
https://matplotlib.org/3.1.1/gallery/shapes_and_collections/hatch_demo.html

tentative API:
```go
package vg/draw

type FillStyle struct {
Color color.Color
Background color.Color
Pattern PatternStyle
}

type PatternStyle uint8
const (
NoPattern PatternStyle = iota
HLinePattern
VLinePattern
HVLinePattern
SlashPattern
DoubleSlashPattern
BackSlashPattern
DoubleBackSlashPattern
PlusPattern
CrossPattern
StarPattern
DotPattern
CirclePattern
BigCirclePattern
)
```