<li>MESA_EXTENSION_OVERRIDE - can be used to enable/disable extensions.
A value such as "GL_EXT_foo -GL_EXT_bar" will enable the GL_EXT_foo extension
and disable the GL_EXT_bar extension.
+<li>MESA_GLSL - <a href="shading.html#envvars">shading language options</a>
</ul>
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