Reflect pipeline outputs as well as inputs, optionally from other stages

* We add an option to reflect inputs from other stages than vertex, if only a
  later subset of the stages is linked into the program.

Test/reflection.vert

@@ -174,6 +174,8 @@ buffer VertexCollection {
     TriangleInfo t[5];
 };
 
+out float outval;
+
 void main()
 {
     liveFunction1(image_ui2D, sampler_2D, sampler_2DMSArray);
@@ -234,4 +236,6 @@ void main()
     f += t[gl_InstanceID].v[gl_InstanceID].position[gl_InstanceID];
     f += t[gl_InstanceID].v[gl_InstanceID].normal[gl_InstanceID];
     TriangleInfo tlocal[5] = t;
+
+    outval = f;
 }