Added tests and documentation for soa<> rate qualifier.

tests/soa-19.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[10];
+    foreach (i = 0 ... 80) {
+        pts[i].x = b*i;
+        pts[i].y = 2*b*i;
+        pts[i].z = 3*b*i;
+    }
+    
+    soa<8> Point * ptr = &pts[6+programIndex];
+    ptr->y = -programIndex;;
+
+    RET[programIndex] = pts[6+programIndex].y;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = -programIndex;
+}