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.frag

@@ -0,0 +1,8 @@
+#version 440 core
+
+in float inval;
+
+void main()
+{
+    float f = inval;
+}