glsl: add support for shader stencil export

This adds proper support for the GL_ARB_shader_stencil_export extension
to the GLSL compiler. Thanks to Ian for pointing out where I need to add things.
diff --git a/src/glsl/glsl_parser_extras.cpp b/src/glsl/glsl_parser_extras.cpp
index 844a746..1337bed 100644
--- a/src/glsl/glsl_parser_extras.cpp
+++ b/src/glsl/glsl_parser_extras.cpp
@@ -203,6 +203,14 @@
       state->EXT_texture_array_warn = (ext_mode == extension_warn);
 
       unsupported = !state->extensions->EXT_texture_array;
+   } else if (strcmp(name, "GL_ARB_shader_stencil_export") == 0) {
+      if (state->target != fragment_shader) {
+	 unsupported = true;
+      } else {
+	 state->ARB_shader_stencil_export_enable = (ext_mode != extension_disable);
+	 state->ARB_shader_stencil_export_warn = (ext_mode == extension_warn);
+	 unsupported = !state->extensions->ARB_shader_stencil_export;
+      }
    } else {
       unsupported = true;
    }