glGetString(GL_VERSION). Valid values are point-separated version numbers,
such as "3.0". Mesa will not really implement all the features of the given
version if it's higher than what's normally reported.
+<li>MESA_GLSL_VERSION_OVERRIDE - changes the value returned by
+glGetString(GL_SHADING_LANGUAGE_VERSION). Valid values are integers, such as
+"130". Mesa will not really implement all the features of the given language version
+if it's higher than what's normally reported. (for developers only)
<li>MESA_GLSL - <a href="shading.html#envvars">shading language compiler options</a>
</ul>
<li>MESA_GAMMA - gamma correction coefficients for red, green, blue channels
<li>MESA_XSYNC - enable synchronous X behavior (for debugging only)
<li>MESA_GLX_FORCE_CI - if set, force GLX to treat 8bpp visuals as CI visuals
-<li>MESA_GLX_FX - set to either "fullscreen" for full-screen rendering,
- "window" to render into a window, or "disable" to disable the Glide driver.
<li>MESA_GLX_FORCE_ALPHA - if set, forces RGB windows to have an alpha channel.
<li>MESA_GLX_DEPTH_BITS - specifies default number of bits for depth buffer.
<li>MESA_GLX_ALPHA_BITS - specifies default number of bits for alpha channel.
<li>SOFTPIPE_DUMP_GS - if set, the softpipe driver will print geometry shaders
to stderr
<li>SOFTPIPE_NO_RAST - if set, rasterization is no-op'd. For profiling purposes.
+<li>SOFTPIPE_USE_LLVM - if set, the softpipe driver will try to use LLVM JIT for
+ vertex shading procesing.
</ul>