Fix issue with separable shader validation in iomapper

Test/iomap.variableOutBlockIn.vert

@@ -0,0 +1,11 @@
+#version 440
+
+layout(location = 0) out vec4 a1;
+layout(location = 1) out vec2 a2;
+
+void main()
+{
+    a1 = vec4(1.0);
+    a2 = vec2(0.5);
+    gl_Position = vec4(1.0);
+}