Added tests and documentation for soa<> rate qualifier.

tests/soa-26.ispc

@@ -0,0 +1,28 @@
+
+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];
+    for (uniform int i = 0; i < 32; ++i) {
+        pts[i].vec.x = b*i;
+        pts[i].vec.y = -b*i;
+        pts[i].vec.z = 2*b*i;
+        pts[i].z = i;
+    }
+
+    RET[programIndex] = pts[9].vec.y;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = -45;
+}