intel: Add a INTEL_DEBUG=color option.

This will be used for color output in debug messages.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

docs/envvars.html

@@ -160,6 +160,7 @@ See the <a href="xlibdriver.html">Xlib software driver page</a> for details.
    This is useful for debugging hangs, etc.</li>
 <li>INTEL_DEBUG - a comma-separated list of named flags, which do various things:
 <ul>
+   <li>color - use color in output</li>
    <li>tex - emit messages about textures.</li>
    <li>state - emit messages about state flag tracking</li>
    <li>blit - emit messages about blit operations</li>