Added tests and documentation for soa<> rate qualifier.

tests/scatter-struct-with-array-member.ispc

@@ -0,0 +1,30 @@
+
+struct Point { float x, y[3], z; };
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    float a = aFOO[programIndex]; 
+
+//CO    soa<8> Point pts[10];
+    uniform Point pts[80];
+    foreach (i = 0 ... 80) {
+        pts[i].x = b*i;
+        pts[i].y[0] = 2*b*i;
+        pts[i].y[1] = 2*b*i+1;
+        pts[i].y[2] = 2*b*i+2;
+        pts[i].z = 3*b*i;
+    }
+
+    a *= -1;
+    Point vp = { a, { 2*a, 3*a, 4*a }, {5*a} };
+    pts[2+programIndex] = vp;
+
+    RET[programIndex] = pts[programIndex].y[2];
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = -4 * (programIndex-1);
+    RET[0] = 2;
+    RET[1] = 12;
+}