nvc0: add support for GL_EXT_window_rectangles

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>

docs/relnotes/12.1.0.html

@@ -46,6 +46,7 @@ Note: some of the new features are only available with certain drivers.
 <ul>
 <li>GL_ARB_shader_group_vote on nvc0</li>
 <li>GL_ARB_ES3_1_compatibility on i965</li>
+<li>GL_EXT_window_rectangles on nvc0</li>
 </ul>
 
 <h2>Bug fixes</h2>