add cross-stage check for missing outputs

If an 'in' is present in a shader stage, make sure a matching 'out'
is present in the previous stage. Only enabled when doing Vulkan.

This commit also fixes a bug where previous stage's linkerObjects
got polluted with 'in' variables from the next stage when merging
linker objects.

Test/iomap.crossStage.vk.2.geom

@@ -0,0 +1,32 @@
+#version 460
+
+layout(points) in;
+layout(triangle_strip, max_vertices=3) out;
+
+// Not written by vertex shader
+in vec4 vgo1[];
+
+in vec4 color[];
+
+out vec4 colorOut;
+
+in VV
+{
+	vec4 val;
+} vv[];
+
+out VV2
+{
+	vec4 vv2Val;
+};
+
+void main()
+{
+    for (int i = 0; i < 3; i++) {
+        colorOut = color[i] * vv[i].val;
+		vv2Val = vec4(1.0);
+        EmitVertex();
+    }
+    EndPrimitive();
+}
+