[cairo] Solid color pattern constructors

[Bill Spitzak]

> +    cairo_color_t color;
> +
> +    _cairo_restrict_value (&red,   0.0, 1.0);
> +    _cairo_restrict_value (&green, 0.0, 1.0);
> +    _cairo_restrict_value (&blue,  0.0, 1.0);
> +    _cairo_restrict_value (&alpha, 0.0, 1.0);
> +
> +    _cairo_color_init_rgba (&color, red, green, blue, alpha);

It seems to me that _cairo_color_init_rgba should do the restrict_value. 
  This would allow backends to handle HDR colors if they want, and also 
it should be more efficient and make the results clearer. The following 
code, for instance, is much faster for 0 and 1 and out-of-range and 
handles NaN predictably (turning it into 0):

	if (x >= 1.0) X = 255;
	else if (x > 0) X = (int)(x*255+.5);
	else X = 0;

In addition, I've generally seen "restrict_value" called "clamp" which 
might be a better name. Because 0,1 is a very common range, adding a 
clamp01() function is often a good idea, too.