Added tests and documentation for soa<> rate qualifier.

tests/soa-4.ispc

@@ -0,0 +1,24 @@
+
+struct Point { float x, y, 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> Point pts[8];
+    foreach (i = 0 ... 64) {
+        pts[i].x = 0;
+        pts[i].y = 0;
+        pts[i].z = 0;
+    }
+
+    Point pv = { a, b, -a };
+    pts[8+programIndex] = pv;
+
+    RET[programIndex] = pts[8+programIndex].z;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = -(1 + programIndex);
+}