cairo/canvas.go
func (v *Context) PaintWithAlpha(alpha float64) {
	C.cairo_paint_with_alpha(v.native(), C.double(alpha))
}
func (v *Context) PaintWithAlpha(alpha float64) {
	C.cairo_paint_with_alpha(v.native(), C.double(alpha))
}
// PaintWithAlpha is a wrapper around cairo_paint_with_alpha().