cairo/canvas.go
func (v *Context) SetSourceRGBA(red, green, blue, alpha float64) {
	C.cairo_set_source_rgba(v.native(), C.double(red), C.double(green),
		C.double(blue), C.double(alpha))
}
func (v *Context) SetSourceRGBA(red, green, blue, alpha float64) {
	C.cairo_set_source_rgba(v.native(), C.double(red), C.double(green),
		C.double(blue), C.double(alpha))
}
// SetSourceRGBA is a wrapper around cairo_set_source_rgba().