func (d *Display) AllocImage(r image.Rectangle, pix Pix, repl bool, val Color) (*Image, error) {
	d.mu.Lock()
	defer d.mu.Unlock()
	return allocImage(d, nil, r, pix, repl, val, 0, 0)
}
AllocImage allocates a new Image on display d. The arguments are:
- the rectangle representing the size
- the pixel descriptor: RGBA32 etc.
- whether the image is to be replicated (tiled)
- the starting background color for the image
package draw
import (
	"fmt"
	"image"
	"runtime"
)