Added tests and documentation for soa<> rate qualifier.

tests/soa-21.ispc

@@ -0,0 +1,32 @@
+
+struct Point { float x, y, z; };
+
+struct Foo {
+    int x;
+    Point pts[10];
+    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];
+    foreach (i = 0 ... 32) {
+        pts[i].x = b*i;
+        pts[i].z = -b*i;
+        for (uniform int j = 0; j < 10; ++j) {
+            pts[i].pts[j].x = j + 100*i;
+            pts[i].pts[j].y = j + 1000*i;
+            pts[i].pts[j].z = j + 10000*i;
+        }
+    }
+    
+    soa<8> Foo * ptr = &pts[7+programIndex];
+    RET[programIndex] = ptr->pts[3].z;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 10000 * (7 + programIndex) + 3;
+}