Full stack: properly implement GL_EXT_device_group and GL_EXT_multiview.

Test/400.tesc

@@ -103,3 +103,23 @@ patch out pinbn {
 
 invariant precise out vec4 badOrder[]; // ERROR, precise must appear first
 void badp(out precise float f);        // ERROR, precise must appear first
+
+void devi()
+{
+    gl_DeviceIndex; // ERROR, no extension
+    gl_ViewIndex;   // ERROR, no extension
+}
+
+#ifdef GL_EXT_device_group
+#extension GL_EXT_device_group : enable
+#endif
+
+#ifdef GL_EXT_device_group
+#extension GL_EXT_multiview : enable
+#endif
+
+void devie()
+{
+    gl_DeviceIndex;
+    gl_ViewIndex;
+}