vg/draw: add a FillStyle

[sbinet]

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:

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
)