fix incorrect error when multiple compilation units don't declare layouts (#2238)

when using multiple compilation units, input/output layouts don't need
to be declared in every unit.

Test/contradict_0.geom

@@ -0,0 +1,13 @@
+#version 330
+
+layout(points) in;
+layout(triangle_strip, max_vertices = 4) out;
+
+float getV();
+
+void main (void)
+{
+    float v = getV();
+    EndPrimitive();
+    EndPrimitive();
+}