Added tests and documentation for soa<> rate qualifier.

tests_errors/soa-15.ispc

@@ -0,0 +1,25 @@
+// Illegal to use binary operator - with SOA type "soa<4> float"
+
+struct Pt { float x, y, z; };
+
+struct Foo { 
+//CO    uniform int a; 
+//CO    int a;
+//CO    float *x;
+//CO    float y[5], z; 
+    double d[5];
+    float x, y, z;
+    //Pt p; 
+};
+
+extern void bar(float);
+
+uniform Foo uf;
+varying Foo vf;
+
+int index;
+
+export void x(uniform Foo &x, soa<8> Foo f[]) {
+    soa<4> Foo xyz;
+    xyz.x = xyz.y-xyz.x;
+}