docs: document more MESA_GLSL env var options
diff --git a/docs/shading.html b/docs/shading.html
index 77c86be..96060f1 100644
--- a/docs/shading.html
+++ b/docs/shading.html
@@ -39,7 +39,8 @@
<p>
The <b>MESA_GLSL</b> environment variable can be set to a comma-separated
-list of keywords to control some aspects of the GLSL compiler:
+list of keywords to control some aspects of the GLSL compiler and shader
+execution. These are generally used for debugging.
</p>
<ul>
<li>dump - print GLSL shader code to stdout at link time
@@ -49,6 +50,11 @@
<li>nopt - disable compiler optimizations
<li>opt - force compiler optimizations
<li>uniform - print message to stdout when glUniform is called
+<li>nopvert - force vertex shaders to be a simple shader that just transforms
+ the vertex position with ftransform() and passes through the color and
+ texcoord[0] attributes.
+<li>nopfrag - force fragment shader to be a simple shader that passes
+ through the color attribute.
</ul>
<p>
Example: export MESA_GLSL=dump,nopt