Added structure alignment in headers; extended the test system to support alignment tests

tests/align2.ispc

@@ -0,0 +1,21 @@
+struct InnerVec {
+    int i;
+};
+
+struct f_sz {
+    InnerVec u;
+    uniform int tail;
+};
+
+
+
+export void keep_struct_declared(varying f_sz * uniform s) {
+}
+
+export uniform int width() {
+    return 1;
+}
+
+export void result(uniform float RET[]) {
+    RET[0] = sizeof(f_sz);
+}