code: func (dc *Context) SetFillStyle(pattern Pattern) {
  	// if pattern is SolidPattern, also change dc.color(for dc.Clear, dc.drawString)
  	if fillStyle, ok := pattern.(*solidPattern); ok {
  		dc.color = fillStyle.color
  	}
  	dc.fillPattern = pattern
  }
docs: // SetFillStyle sets current fill style