Add option to glslangValidator to inject preamble

Test/glsl.-P.frag

@@ -0,0 +1,20 @@
+#version 450
+
+layout(location=0) out vec4 color;
+
+void main()
+{
+    #ifndef TEST1
+    #error TEST1 is not defined
+    #endif
+
+    #ifndef TEST2
+    #error TEST2 is not defined
+    #endif
+
+    #ifndef TEST3
+    #error TEST3 is not defined
+    #endif
+
+    color = vec4(1.0);
+}