Warning for #pragma once. GLSL and HLSL. Addresses #824.

Test/baseResults/preprocessor.pragma.vert.err

@@ -0,0 +1,2 @@
+WARNING: 0:10: '#pragma once' : not implemented 
+

Test/baseResults/preprocessor.pragma.vert.out

@@ -7,6 +7,8 @@
 
 #pragma undefined_pragma(x,4)
 
+#pragma once
+
 int main(){
 }
 

Test/preprocessor.pragma.vert

@@ -7,5 +7,7 @@
 
 #pragma undefined_pragma(x, 4)
 
+#pragma once
+
 int main() {
 }