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_CACHE_DISABLE - if set, disables the GLSL shader cache
+<li>MESA_GLSL_CACHE_MAX_SIZE - if set, determines the maximum size of
+the on-disk cache of compiled GLSL programs. Should be set to a number
+optionally followed by 'K', 'M', or 'G' to specify a size in
+kilobytes, megabytes, or gigabytes. By default, gigabytes will be
+assumed. And if unset, a maxium size of 1GB will be used.
+<li>MESA_GLSL_CACHE_DIR - if set, determines the directory to be used
+for the on-disk cache of compiled GLSL programs. If this variable is
+not set, then the cache will be stored in $XDG_CACHE_HOME/mesa (if
+that variable is set), or else within .cache/mesa within the user's
+home directory.
<li>MESA_GLSL - <a href="shading.html#envvars">shading language compiler options</a>
<li>MESA_NO_MINMAX_CACHE - when set, the minmax index cache is globally disabled.
</ul>