Added tests and documentation for soa<> rate qualifier.

tests/soa-25.ispc

@@ -0,0 +1,32 @@
+
+struct Point { float x, y, z; };
+
+struct Foo {
+    float<3> vec;
+    int8 z;
+};
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    float a = aFOO[programIndex]; 
+
+    soa<8> Foo * uniform pts = uniform new soa<8> Foo[4];
+//CO    uniform Foo pts[32];
+    foreach (i = 0 ... 32) {
+        pts[i].vec.x = b*i;
+        pts[i].vec.y = -b*i;
+        pts[i].vec.z = 2*b*i;
+        pts[i].z = i;
+    }
+
+    pts[2].vec.x *= -1;    
+    float<3> vl = pts[programIndex].vec;
+    RET[programIndex] = vl.x;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 5 * programIndex;
+    if (programIndex == 2)
+        RET[programIndex] *= -1;
+}