Issue #1354 - Don't allow glsl[130,400] unless we have gpu_shader5

- Teach GLContext about gpu_shader5
- Downgrade shader language version if gpu_shader5 support isn't found.

gfx/gl/GLContextFeatures.cpp

@@ -331,6 +331,18 @@ static const FeatureInfo sFeatureInfoArr[] = {
             GLContext::Extensions_End
         }
     },
+    {
+        "gpu_shader5",
+        GLVersion::GL4,
+        GLESVersion::NONE,
+        GLContext::Extension_None,
+        {
+            GLContext::ARB_gpu_shader5,
+            GLContext::EXT_gpu_shader5,
+            GLContext::NV_gpu_shader5,
+            GLContext::Extensions_End
+        }
+    },    
     {
         "instanced_arrays",
         GLVersion::GL3_3,