docs: document 'useprog' debug option

docs/shading.html

@@ -55,6 +55,7 @@ execution.  These are generally used for debugging.
     texcoord[0] attributes.
 <li>nopfrag - force fragment shader to be a simple shader that passes
     through the color attribute.
+<li>useprog - log glUseProgram calls to stderr
 </ul>
 <p>
 Example:  export MESA_GLSL=dump,nopt