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.frag

@@ -0,0 +1,24 @@
+#version 460
+
+in vec4 unsetColor;
+in vec4 colorOut;
+out vec4 fragColor;
+
+in Vertex
+{
+	vec2 val;
+} iVert;
+
+in Vertex2
+{
+	vec2 val2;
+};
+
+in vec4 vv2Val;
+
+void main()
+{
+    fragColor = colorOut + unsetColor * vec4(iVert.val.xxyy) * vec4(val2.xxyy) * 
+				vv2Val;
+}
+