Type convert NULL to other pointer types for function call arguments.

Fixes issue #198.

tests/ptr-null-func-arg.ispc

@@ -0,0 +1,14 @@
+
+export uniform int width() { return programCount; }
+
+bool bar(float * x) {
+    return (x != 0);
+}
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    RET[programIndex] = bar(NULL);
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 0;
+}